Uujima Satoshi

During Irisu Syndrome, Irisu Kyouko is revealed to be in love with a university student named Uujima Satoshi, or "Uuji" for short. Unknown to her, Uuji is the reason for her descent into madness, having been personally responsible for the tragedy of her rabbit.

Background
Uuji is friends with Ageha and Edogawa but hides a sociopathic side demonstrated best by his drawings of tortured cats (done in a style reminiscent of The Book of Bunny Suicides). He encounters Irisu at some point and becomes smitten with her, the attraction being mutual. Uuji learns of her willingness to murder and her intention to kill Ageha and Edogawa for being too close to the only human she seems to trust (especially Ageha for developing a crush on Uuji). Knowing this, Uuji decides to further tempt by arranging his own death by enraging her with a sick caricature of a cute bunny tortured as a kind of depraved "romantic gesture".

It's revealed that he was the one that murdered her rabbit, leaving its head for her to find. His reasoning was apparently to get her to become less shy and less pained by the rabbit's eventual death. This only further demonstrates Uuji's sociopathy. (It is hinted that the rabbit's soul possesses Irisu and is seeking revenge against Uuji for its death and his mistreatment of Irisu.)

Fate
Depending on the player's progress, different endings will result. In the bad ending, Irisu snaps and murders the three friends. In the extra stinger, it is shown that Irisu encounters Uuji's grotesque illustration of the bunny suicide, implying that she will go berserk and murder them. In both of these scenarios, Uuji fulfills his goal and Irisu is left in her insanity.

In the "Metsu" ending however, Irisu is pacified and welcomed into the group as a fourth friend. Uuji is uncertain of whether he's happy with this scenario but goes along with it. However, the ending implies that someone found his journal with the bunny suicide illustration, implying that she may commit murder either way.