Seeing as there is no English version yet, I decided to do the community a favor for all the Vanilla, gag and troll mangas. I translated it, not on my own, but using the Japanese and chinese versions along with the finnish version to crosscheck. There might still be mistakes, and I am very sorry if I translated the crucial parts wrong (namely, the parts involving the evidence etc.).

Credits go to Graus for the original scans. MrSake & Punakone for the providing of the doujin and the finnish translation and to pan712 for the chinese translation.

Enjoy!

Wow...when showed Patch's dead body, I almost cried t_t

but the story reallys interesting, good questions, methods of thinking...I got really deep into it.

Would love to see more stories of this type ;3

Base +12, Akiyama Ikuto +3, ShatteredWings +14, Vermiurge +4, jfragrettel +5

I am weak against this. Who did it and how?

Base +4, Vermiurge +4, jfragrettel +5

...Meiling didn't know the master key was broken by Flandre. Yet Aya confirm her alibi since Aya also confirms she arrived at 5. It's unlikely the culprit can be either of them.

Actually, there's something wrong with times here. Aya and Sakuya say they started talking half past 6 for one hour, then Marisa arrived 30 minutes later. Yet the timeline shows they started talking half past 5. Toot Oriole, go recheck your translation because something is clearly wrong here.

Base +10, Bne -6, Vermiurge +4, jfragrettel +5

@megamano, Thanks, I noticed that too.

I always mess up with the time when they say "half 6, (puoli kuusi) which means half past 5" in finnish, sounds simple if you listen closely, but for me, I tend to mess it up and once I ended up being an hour late from a meeting~
I also blame this issue because I stayed up until 2AM to finish it~ :'D

EDIT: @sayasaya, I can confirm it is not a mistake, Meilings kanjis are present in the original japanese version, so I simply did my job~

Base +7, Vermiurge +4, jfragrettel +5

Witches aren't real, so it had to be small bombs in Patchy's food that did it.

Base +9, PhantomRenegade +9, hjhj210 +11, Ilosia +11, Rei-Tenshi +29, Violentin +7, jacksonville +16, Tooolsss +8, StarfishHeat +7, BlickWinkel +8, Akiyama Ikuto +3, Refon_S +7, ShatteredWings +14, and 18 more...

Logic puzzles. My one greatest weakness.

Base +6, PhantomRenegade +9, Krugger +7, Rei-Tenshi +29, Tilly +7, Akiyama Ikuto +3, Refon_S +7, ShatteredWings +14, Dottboy +11, sauceguides +8, Anonyslime +8, kaylie +7, Marisa Kirisame +6, and 4 more...

Do my eyes deceive me? It's this real? I'm not dreaming? It's finally in English!

Base +9, ShatteredWings +14, Dottboy +11, Vermiurge +4, jfragrettel +5

I think it was Sakuya.

She could have lied when she said that the door, leading to the library was locked. She also took Patchi's body to her room to make it look like someone closed both doors. I think she also lied when it comes to the Master Key, which she could have just hidden all the time. (we never see the broken key)

The only problem is the timespan. Patchi died at 17:00, but Sakuya got interviewed 17:15. Reimu clarified that it takes 20 Minutes to go from the Library to Remilias Chanmber.

Base +8, ShatteredWings +14, Vermiurge +4, jfragrettel +5

Where the hell is Koakuma?

Base +5, Krugger +7, Akiyama Ikuto +3, toutorp +1, ShatteredWings +14, sauceguides +8, FLCL_hunter +7, kathi9000 +4, Watsa +9, herpderp5545 +7, jfragrettel +5

On pg22, it's either translation mistake or...when Sakuya said that Mei Ling can confirm that the library door was locked, yet the only one who touched the door was Sakayu - Marisa broke it open. Then there's meiling doubting that sakayu about the master key during interrogation.
THIS MEANS ONE THING! Dun dun dunnnnn.
We need the sequel, please!

Base +2, Vermiurge +4, jfragrettel +5

I think I've solved it.

Solution 1:
Marisa, Reimu, Eirin, and Yukari are all ruled out because Meiling would have seen them enter. Aya is simillarly ruled out because she was talking with Meiling when the murder occured. Aya confirms this, and so Meiling can't be the killer either. Patchouli couldn't have commited suicide because the weapon was in a different room. It can't be Sakuya because she wouldn't have been able to go from the library to Remilia's room in the 15 minutes before Aya got there, and Remillia couldn't have left her room without Sakuya noticing.

Solution 2:
The killer had the master key. After killing Patchouli, she moved her to her bedroom and locked both doors with the master key.

Solution 3:
The murder was an accident, and the murderer didn't realize that Patchouli was dead after that blow to the head. She assumed that Patchouli was merely unconscious, and so laid her in her bed and locked the library and bedroom doors, just like Patchouli would have done if she were going to sleep. She left the keys in the room because she didn't want Patchouli to be trapped in her room when she woke up.

Solution 4:
Flandre is the killer.

In the commotion at noon, Flandre broke Sakuya's master key and secretly kept the end bit, so Sakuya only had the handle. Then later, she used that bit to sneak out of her locked room and went to see Patchouli. They got into a fight, and either Flandre got mad and threw a book or Patchouli threatened to tell Remilia that Flandre was out of her room and Flandre tried to knock her out. The book was blunt and so left a wound that didn't bleed much, but did a lot of internal damage (which is why there's a light blood trail to the bedroom despite the fact that she was hurt enough to die immediately). Flandre didn't notice the small amount of blood because both her dress and the floor are red. The book was brushed against Flandre's dress when she put it back on the table, which explains the lack of intact fingerprints and why it has so much blood on it.

Base +8, 95298008 +4, Tilly +7, Akiyama Ikuto +3, cokerpilot +7, Bne -6, werty101 +9, 34se -7, ShatteredWings +14, randomchakra -9, Dottboy +11, wavedash +10, dementio +7, and 3 more...

It was Tokiko.

Base +5, Wizbro +6, Vermiurge +4, jfragrettel +5

PATCHY KILLED HERSELF

SHE WAS TURNING INTO A ZOMBIE OR SOMETHING AND DIDNT WANT TO HURT ANYONE ELSE BEFORE LOSING HER MIND

Base +6, Marisa Kirisame -6, Vermiurge +4, jfragrettel +5, tubo +5

@Person8
Beware of Marisa now. She won't give up that easily.

@m42a
Isn't that just way too easy? A girl in a separate room nobody kept an eye on? True, Flandre could do whatever she pleased should the part of the broken key be in her hands. But this is just so simple I can't believe it.

On the other hand, it is said that Sakuya was ordered to guard Remilia's room throughout her sleep time, yet it wasn't clearly stated whether ze maid really did so, except for the interview time with Aya. Sure, it has been proven that Sakuya can't be the culprit simply because she coulnd't get back to the Remi's room in time (And time-stopping ability is negated by the rules of this mystery).
Despite this, after checking on her mistress Sakuya could move away for a short while, for whatever reason, thus giving Remi an opportunity to sneak away. Later on, she returned back to her room during the body discovery ruckus, when everyone else gathered downstairs. Could this be the case?

Base +5, Vermiurge +4, jfragrettel +5

Personally, I'm going to go with what I've learned from reading and solving mysteries in my youth:

This is the first one of these I've seen. Therefore, the culprit can be safely assumed to be Bugs Meany.

Base +8, werty101 -9, wavedash -10, Marisa Kirisame -6, Vermiurge -4, jfragrettel -5

Problem One: Cant explain, too lazy to think

Problem Two: She was the one who checked the sealed room and declared it locked. No other character bother to checked and instantly broke through the door.

Problem Three: Patchouli isn't really dead and it is just a ploy created by those two in order to get a mystery going.

Problem Four: Marisa Kirisame

Base +5, Vermiurge +4, jfragrettel +5

Lets look at the facts.

Flandre is ruled out immediately. She wouldn't bother making it look like a sealed room murder, since she's a lunatic to begin with. She'd just rip Patchy to shreds and leave here there, not caring about covering it up. Patchouli would also be wary of Flandre due to Flan's violent nature, so the crime scene would have shown more violence.
People Patchouli trust, like Sakuya, Meiling and Remilia, would be able to take her by surprise, resulting in a murder with little struggle. There were no finger prints on the book, so the murderer must have used gloves. Flandre wouldn't have bothered wearing gloves, and if she murdered Patchy on the spot after escaping Flandre wouldn't be wearing gloves either.
Saying Flandre used a broken half of the master key to open the door wouldn't be viable proof, as we don't know in what way the key was broken. It could have been broken in several small pieces for all we know. We do not see the broken master key either.
The murder was committed with finesse, and planned out beforehand, as stated a couple of times in the doujin. Flandre would never bother to plan out a murder. She'd just kill the person right away and be done with it.

Reimu, Yukari, Eirin and Marisa are ruled out, as Meiling stated nobody arrived at the mansion before 20.00 except for Aya, who arrived at 17.00 and interviewed Meiling till 17.30 and Sakuya till 18.30. Patchouli was murdered at 17.15 according to the time table on page 41. Since Meiling was being interviewed by Aya from 17.00 to 17.30, neither Aya nor Meiling could be the culprit.

It takes 20 minutes to get from Remi's bedroom to the library. To go from the bedroom, commit the murder, set the scene up and then get back to the bedroom would take more than 40 minutes. The murder was committed at 17.15, and Aya interviewed Sakuya at 17.30. Sakuya checked up on her sleeping Mistress at 16.20. This gives Sakuya a full hour and ten minutes to go to the library, murder Patchouli, make it look like a sealed room murder and then get back in time for the interview with Aya.
Sakuya also never shows the broken master key, and Meiling mentions that Sakuya could very well be lying about the key being broken. Sakuya, being the one with the master key, could have used it to make the murder look like a sealed room case. Sakuya has the intelligence and finesse to set up such a murder, as well as having the common sense to use gloves to avoid finger prints getting on the crime scene. Sakuya tried to cover it up to keep up her image as a perfect, elegant, loyal maid to her Mistress. Sakuya also lies about the times, saying she took care of Aya at 17.30 and that Marisa arrived at 18.00, both of which are incorrect according to the time table and the other characters' cases. Sakuya also doesn't finish her last sentence during her case, which is very suspicious.

All these facts would make Sakuya look like the murderer, but there is one thing that rules her out: Marisa stated that the murderer would be covered in blood after the murder, and that the murderer would be wearing clothes on which blood stains would be hard to see. Sakuya wears a blue and white maid uniform, so blood stains would be very easy to spot on her outfit. Thus, Sakuya couldn't have been the murderer.

The only characters with clothes on which blood would be hard to see are Remilia, Flandre, Eirin and Reimu. Marisa's white apron would easily be bloodied, same goes for Sakuya's apron, Yukari's white dress and Meiling's green china dress. Reimu, Marisa, Aya, Yukari, Eirin, Meiling, Sakuya and Flandre have already been ruled out. So that leaves... Remilia...

Remilia stated she was sleeping in her bedroom from 16.00 till after 20.00, but nobody has checked on her if she really was sleeping except Sakuya. Sakuya might not have assisted with the murder, but she could very well have helped make an alibi for the Mistress. Remilia got the master key from Sakuya and went to the library, murdering Patchouli at 17.15. Patchouli trusts Remilia, so there was little struggle during the murder, and bloodstains wouldn't be noticeable on Remilia's already blood-soaked dress. Remilia is clever and refined enough to set up a sealed room murder and use gloves, and would set up such a murder to maintain her publicity as a refined and noble lady. Remilia could have hid away somewhere in the SDM while Aya was around. The mansion is big enough to have loads of hiding places.
After Aya left, Remilia returned to the bedroom to act as if she was asleep all along. Sakuya tried to cover herself and her Mistress up by giving false times, by saying the master key was broken and that her Mistress was asleep the entire time. Sakuya could also have been merely mistaken about the times though.

All these facts point to Remilia, so in conclusion, Remilia is the murderer.

EDIT: Remilia could have also taken the master key during the Flandre fight, and have told Sakuya that the key was broken. Sakuya blindly believes her Mistress, allowing Remilia to use the key to set up the sealed room murder. Remilia could have lied about her sleeping to Sakuya as well, using a hidden passage in her room to leave the bedroom after Sakuya checked up on her. That would make Remilia the only liar, and the only murderer.

Base +1, dreamsagexna +8, savaris +6, 34se -7, ShatteredWings +14, Yuyu90 -4, Dottboy +11, ryuho209 +9, jfragrettel +5

Okay, SPOILER ALERT! DO NOT READ UNLESS YOU WANT TO KNOW THE MURDERER!
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.....
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The murderer is Marisa. How do I know? I read the answer. http://www.mizuryu.jp/pacheckxlogic/kaiketsu.html

Meiling did fall asleep, for approximately 1 hour. During which Marisa had sneaked in. She killed Pathchy, took the library key with her, and went home to change before coming back at 18:00.

Flandre is not the killer. She's not even an option. The murderer was interviewed.

Sakuya could not have killed her and made it back in time for the interview.

Remillia was checked on by Sakuya at 6:20. I don't know if there's a translation error somewhere but she was supposedly checked on every 20 minutes, not just the first 20 minutes. She has been ruled out.

Meiling fell asleep. And was interviewed by Aya during the time of the murder.

Aya was interviewing Meiling during the time of the murder.

Base +8, 34se +7, ShatteredWings +14, randomchakra +9, Dottboy +11, sauceguides +8, Ziginus +7, Homi +7, dementio -7, Lashauk +9, Marisa Kirisame +6, Vermiurge +4, janusi +12, and 2 more...

Well, thinking about it, let's go about this by the assumption that only the culprit may lie. That's the feeling I'm getting from Fact #1:, but whether it's because of translation or the author meant it that way, it's bloody ambiguous. Using this assumption and the Fact #4 that there is only one culprit, this quickly becomes a logic puzzle.

Culprits: Patchouli, Remilia, Sakuya, Meiling, Reimu, Yukari, Aya, Marisa, Flandre(?)

Patchouli: A suicide is not possible due to an instantaneous death. Fingerprints must have been removed from the murder weapon after the crime, which is not possible if she is already dead.

Remilia: Remilia provides an alibi between 16:00 to approximately 20:00, with Sakuya as witness. Since Sakuya checked up on her at 16:20, Remilia did indeed prove the first part of her alibi. A gap exists between 16:20 to 17:30 in Sakuya's alibi for her guarding Remilia's door, however. Thanks to Aya and Sakuya's alibi, the interview took place in front of Remilia's room from 17:30. Discrepancy in where it ends, but I am assuming the interview lasted until 18:30, while an hour of negotiations took place from 18:30 to 19:30. This would satisfy both characters' alibies.

Now then, the problem lies: we cannot confirm Remilia's actions from 16:20 to 17:30. Remilia sneaks off during this time, and commits the murder. She has more than two hours from the murder until its discovery, so she has ample time to set the scene up, and she's proven to be intelligent. Also, Sakuya is absent from Remilia's door from at least 19:40. Remilia hides during the time when Sakuya is on guard, sneaks off and enters her room and feigns sleep until she is called for either by Sakuya or a fairy maid some time after 20:00.

Her alibi for 17:30 to 19:30 is foolproof, but that doesn't help. Remilia definitely has a possibility of being the suspect.

Sakuya: Easily the most suspicious one by far. And a red herring, no doubt. The most crucial part of her alibi requires that she be at Remilia's room by 17:30. Crime was committed at 17:15, but the distance takes a minimum of 20 minutes. She therefore cannot have committed the crime.

Meiling: Also foolproof. She and Aya both directly support each other during 17:00 to 17:30; either both of them lies or they both tell the truth, and due to Fact #4, they are innocent.

Reimu: She cannot be the culprit as she arrived at 23:00, much later than the time of the crime. Meiling is a witness to this alibi.

Yukari: See Reimu. Her gap magic also cannot be used a tool for murder due to Fact #3.

Aya: See Meiling.

Marisa: See Reimu.

Flandre: Flandre is... a tricky case. Thanks to almost nil information we receive about her, she has little room to support herself. However, Fact #4 is our friend here. Flandre has never been presented in the story as a character. She has been mentioned, but we never see her on-stage. However, assuming that counts as being presented, she fails several character checks. Patchouli letting her guard down near Flandre is very doubtful - especially when Flandre was making a mess out of everything just a few hours earlier. Flandre having the finesse to clean up her act is very doubtful. Flandre being capable of setting up the closed room is very doubtful, especially as youkai powers (vampirism included) is disabled. She is unlikely to be the culprit.

So excluding Flandre, we have a culprit to work with. Remilia Scarlet. I think that takes care of Solution 1.

Base +2, Vermiurge +4, jfragrettel +5

I am not really sure whether i am right or not. But i am sure of one thing. Flandre is NOT the culprit.

Ignoring her so-called psycopathy tendency (when i play EoSD she seems a cute normal girl), there are several reasons.

The first is about Blood stain. Among the suspect noted about blood that could have splashed to the suspect body. But i am quick to dismiss Flandre as the culprit due to her scarletish red color that she fond to wear. Why? it seems plausible that if due to autopsy record which the victim died immediately due to blood loss, and it splattered to the culprit who used book as her tool, then Flandre Scarlet is The First to be absolved as the suspect due to her clothes color.

Due to the Detective (Reimu) late arrival, three hours... the Post Mortem condition of the victim's body would have changed, and even if the Locus delicti remain unchanged, then the body condition will. For example , A dried Blood or Blood Stain would change color due to time and temperature, seeing three hours would be more than enough to turned the reddish Hb into a dark-red or more similiar to Black color. hence, if suspect Flandre really killed the victim using book, then her red clothes will leave a black stained color instead of RED! so the detective would know immediately.

This fact was supported when the Watson (Yukari Yakumo) found a blood stain in the red carpetted room, while the other character noted that they had not seen them before.

While the testimony of all the culprit are implausible, it is hard to determine whether they are all telling the truth.

First suspect Remilia Scarlet tried to prevent her sister Flandre from being interrogated. this is already suspicious as if Remilia is afraid that Flandre know something.

Second Suspect Sakuya Izayoi has no witness who can support of her alibi of guarding Remilia Scarlet after 18.30 to 20.00, enough time to commit the murder

Third Suspect Hong Meiling is LYING. she fell asleep during her duty, so her testimony is NULL. Supported by suspect Aya Shamameiru who claimed witnessing her drowzy. she must have missed someone sneaking behind her.Her attempt to make Sakuya Izayoi suspicious is also an attempt to avoid the invalidity of her testimony.

Fourth Suspect Marisa Kirisame absolves suspect Flandre Scarlet.

Fifth Suspect Aya Shamameiru.... there is no one who can support her alibi after 18.30

The time is death is 17.15, so Suspect Remilia Scarlet, Sakuya Izayoi, Aya Shamameiru is absolved.

The only possible culprit is Marisa Kirisame. No one can support her alibi prior 20.00 who claim she is at her own home.

She could have taken advantage during the confusion involving Flandre occurred to sneaked inside the Scarlet Devil Mansion from the Main gate without anyone noticing (Hong Meiling's testimony is Null due she is sleeping in her duty). She arrived earlier from the promised time and later an accident or incident occurred. 17.15 the victim dies inside library. culprit Marisa later brought the victim to her room and locked both doors. She later return home.

20.00 she return there as if she had not come earlier, and during the time the victim's found, she could have put the key or pretended to find the key of both room. three hours of time is more than plenty time of preparation and act.

The culprit has no partner but she could have also let another suspect to act without their own consciousness in finding the key while being distracted by the founding of the victim's body.

Finally.... Alice Margatroid is involved in this crime.

Base +8, snipkun +7, Vermiurge +4, jfragrettel +5

Ok, my thoughts in it

1 - Pretty much said here: Reimu, Yukari, Marisa and Eirin didn't go inside before the time murder, Meiling, Sakuya and Aya don't used any power so there's no way to reach the room so fast and go back to their places like the time says

2 and 3 - It's kinda hard when you are in doubt about the master key being broken or not... but believing the rule "there's no partner", only the killer lied so I will believe there's no master key involved. The killer murdered her in the library, took the body and locked the room, so the killer was inside the room when they met the body, easily hidden somewhere like under the bed.

When they found the body, Marisa, Sakuya and Meiling was together. Aya like before can't kill her and go talk to Sakuya without flying.

4 - And Remilia only appeared after Reimu and Yukari was called!
My only problem is Sakuya, she said she saw Remilia sleeping, but did she really saw her or saw a volume under blankets thinking she was there?
Maybe there's a time where Remilia sneak out of her room. And after they bust open the door of the sealed room she could sneak her way out and go back to her room!

damn... too much Umineko brings something!

EDIT: checked the answer given in the comments, with the mistranslation of Sakuya checking her each 20 mins my logic is all broken! Goddammit, just when I thought I was being smart!

Base +6, Vermiurge +4, jfragrettel +5

I can't read Japanese, but through a combination of Google Translate and a bit of guesswork, I can elaborate a little on what Auir posted. As with their post, SPOILER ALERT!!!
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An important part of the Marisa-is-the-killer solution is that Meiling was covering up the fact that she fell asleep (that's also why she was surprised to hear that Sakuya didn't have her key; it wasn't an attempt to incriminate Sakuya, Meiling just slept through Flandre's rampage). She never "found" the key at all, she had already had it in her possession and just pretended to find it. She's the one who moved Patchouli's body to her bed, and locked the bedroom door. Marisa was… hiding nearby, or something, and locked the library door after Meiling had moved the body, then later "found" that key too.

Frankly, I don't know why bringing Patchouli to her bed was supposed to be any kind of a cover-up, but that's how the official solution explains the double locked doors. Is there any chance for somebody to translate it properly so we can all read it?

Base +11, ShatteredWings +14, Dottboy +11, sauceguides +8, kathi9000 +4, Marisa Kirisame +6, jfragrettel +5

The only thing bothering me here is that "Akasha" is written in Arabic letters on Pachy's book. It should probably be in Devanagari script, being a Sanskrit word and all.

Base +6, Vermiurge +4, jfragrettel +5

SPOILER ALERT DON'T READ IF YOU WANT TO FIND THE CULPRIT BY YOURSELF !
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so yeah thank to http://www.mizuryu.jp/pacheckxlogic/kaiketsu.html we know that's is marisa the murderer but we still don't know
WHY ??? what's the motive ffs ? to me that crucial to know this fact

Base +2, Vermiurge +4, jfragrettel +5

Those who keep saying Marissa is the murdered... okay, let's say she is.

Why and how she did this? Why, the narration from the start to the end, from her point, make her sound like she has no idea of this? Why would she narrate, in her head, everything as if she had no idea?

In short, don't just say Marissa did it based on images, give us something more!

Base +6, Vermiurge +4, jfragrettel +5

So, wait, quick theory here:
Sakuya did it.
Let's see:
Time of death: 17:15
Meiling was interviewed during that time, until 17:30
Sakuya was interviewed after that, until 18:30
She had the time for it, seeing as Remilia went to sleep at 16:00. After twenty minutes, Sakuya checked on Remilia, who was sleeping, making the time 16:20.
So, she had a window of 1 hour and ten minutes, more than enough.
Furthermore, Sakuya is lying about the key. Her master key never broke. Think about what Meiling said: "Did she really not have it?"
Her question has a reason, because if Flandre went crazy at around 13:00, creating a rucus in which Sakuyas key appearently broke, how could Flandre be locked in her room?
If she is lying and still has the key, she could have killed Patchi, carried her to her room (or even killing her in her room, just carying the book to the library, no risk of getting blood on her) placed the keys in Patchis room and locked both rooms with her master key. Furthermore, as the head maid, she would have the equipment to clean a book of finger prints, or even using gloves without looking suspicious.

EDIT: I just remembered that Reimu tested the theory, and she did not manage to get to the library from Remilias room in under 20 minutes. At this time Aya already started the interview with Sakuya.

Base +6, jfragrettel +5

>That artist Tag

I half expected Everyone to turn into a sex-crazed whore at some point, possibly turning this into a creepy house of Sodom tier blood orgy.

Base +7

It's obvious Marisa is the killer.

Assumption
Shikieiki ruled that anybody, aside from the Investigators, may be lying.

Facts:
1. Time of death: 17:15
2. Killer does not have partner
3. Flandre's information and whereabout is too little and I don't believe the author is trying to pin down the mistakes on her.

A. Explain why other characters cannot be the killer:
a. Mei Ling and Aya was ruled out, because both has strong alibi AND Shikieiki's golden rule that the culprit does not a partner.
b. Remilia was ruled out because of Sakuya's guarding her room at the time of murder.
c. Sakuya was interviewed at 17:30. Reimu herself has stated it will take 20 minutes between her place and place of murder.
d. Flandre is not possible because, well, the story.

B. How the culprit make it like a sealed room?
1. Marisa sneaked in when Mei Ling falls asleep sometime before 17:00;
2. At 17:15 inside the main library, Marisa killed Patchouli (after having argument perhaps) with the hard covered book nearby. It is possible this is NOT a planned murder;
3. She then dragged Patchouli into her chamber. Unknown to Marisa, some of the blood stained onto her clothes (black colored);
4. She then search for the Library key. It is possible that Marisa was under pressure that any SDM members may come to visit Patchouli after the deed. She did not realize that there are TWO library keys;
5. She locked the main library door, enter Patchouli's room, and leave the key on her corpse;
6. She then run away (through the back door, obviously) and wait until the appointed time;
7. The 2nd time she came (with Sakuya and Mei Ling), she found out that the book is drenched in blood. Marisa was THE ONE WHO DECIDED TO SPLIT UP AND SEARCH FOR PATCHOULI.
8. Lastly, Marisa was also THE ONE WHO CLAIMED THE DOOR TO PATCHOULI'S CHAMBER WAS LOCKED, and the door was quickly blown away by Mei Ling IMMEDIATELY WITHOUT ANY OF THE 2 CHECKING UP FIRST.

C. Why the culprit make it like a sealed room?
There are 2 explanations:

1. Unplanned murder. Marisa probably trying to make as if Patchouli committed suicide. But by the time she found out that the book is bloodied, she has to make as if Patchouli was killed in a locked room, to pin the mistake onto SDM's personnel (and she is just an outsider who happen to have an appointment).
2. Changing the colour of blood. It is fact that blood will turn colour into darker color after it is oxidized. The hint would be Marisa's cloth. Patchouli's blood must be there.

D. Who is the culplrit?
Marisa Kirisame

E. Motive?
Highly possible it is in relation to Flandre and about the possible Christmas event.

Base +3