I had a moment of time, so I figured I’d take another look at this.
Previously, I ended up going through all kinds of detours because I was so convinced that only people directly shown in the magic scenes would be viable culprits. However, that was, honestly, a pretty stupid thing to assume. So, let’s fix that mistake here and now.
Everyone is in-character and there’s only one person with a simple opportunity to unlock the door to Ritsuka’s room and who might have the motive and character to murder him. Granted, this means that Maria has to be an accomplice of sorts, but considering how much Maria adores Beatrice, that is not a stretch by any means.
The culprit is, of course,
George Elizabeth Bathory.
The true nature of the incident is a murder mystery play that was exploited by Elizabeth to murder Ritsuka because she resented Ritsuka for putting her in the back ranks (in a mail bikini, even) when she’s a proud noble who deserved better. She might have also been jealous of Mashu for being Ritsuka’s favourite even though she’s some commoner. During the night, Mashu went into Ritsuka’s room and asked him to participate in creating a murder mystery play. Ritsuka (and Robin) agreed to this, and set up the closed room from the inside and played dead. Mashu, needing enough actors for her play to succeed, also recruited Nitocris and Elizabeth to participate. Including Elizabeth, however, was a fatal mistake.
Elizabeth decided she’d use this opportunity to murder Ritsuka for real. After Ritsuka and Robin had done their preparations for the locked room, she sneaked, incorporeal, into their bedroom and killed Robin. She had no particular grudge against Robin, but leaving him alive would disturb her plans to kill Ritsuka later, so she had to get rid of him. The next morning, things proceeded as Mashu had planned for his fake murder mystery, with the exception of Robin being actually dead instead of just hiding. Mashu “examined” Ritsuka’s “corpse”, and declared him dead. Naturally, at this point, Ritsuka was just playing dead. Mashu took the key to the room, and everyone went downstairs, Ronove locking the door behind them. When they were downstairs, deciding how they should search the mansion, Elizabeth volunteered to watch the door with Maria, as planned by Mashu, and Mashu gave the key to Ritsuka’s bedroom to her.
Regardless of whether Ibaraki had also been recruited into the plan to lead the other cousins away from the door or whether Mashu had just counted on managing to convince the rest to search the ground floor, Elizabeth and Maria were alone when Ronove, Mashu, Nitocris and Battler were searching the first floor. This gave Elizabeth an opportunity to go upstairs, open the door to Ritsuka’s bedroom, leave with Ritsuka and lock the door behind them, creating the illusion of the vanishing corpse. Elizabeth lead Ritsuka downstairs into some room she knew the ground floor investigation group had already searched, and murdered him there. For a Servant such as her against an unsuspecting ordinary human, killing Ritsuka quickly and discreetly would not be difficult. After she had done the deed, she returned to Maria, who had been watching the door and agreed to keep quiet about Elizabeth leaving for a moment.
Nitocris and Mashu followed Mashu’s murder mystery script, pretending that the witch had taken the corpse upstairs and absconded with it through the door, unaware that Ritsuka had died by Elizabeth’s hand mere moments ago on the ground floor.
That should sufficiently explain the entire case, I hope. My apologies for taking so long.