Not much is really known about Irisu save that she was distant from other children during her middle school days. She particularly bonded with a neglected school rabbit, which helped to develop her obsession with rabbits like that seen of the fake rabbit ears sewn to her hat. This led to her breaking emotionally and mentally once the rabbit was killed. She withdrew but later encountered and fell in love with Uujima Satoshi. (Rather sickeningly, they particularly bond over Uuji's sadistic images of cats being tortured, although her hatred of cats stemmed from finding her rabbit's head being licked by a cat.)
When enraged, she gains reddened eyes and menacingly threatens with a sock filled with rocks and a hardened loaf of bread embedded with nails. She uses the former weapon to kill if the player's progression leads that way.
Diary entries reveal her as having a difficult time relating to people, either due to feeling truly alone or seeing them annoyances she wishes death upon.
In the initial version ("normal" mode) of the game, Irisu is an unusual figure presiding over the player's process in the puzzle. If the player gets too low of a score, she cries while she becomes menacing should the player exceed 400,000 points, complete with red eyes staring.In "Metsu" mode, more of Irisu's background, as indicated above, is revealed as is her relationship with Uuji. Feeling unworthy of his seeming support, she nevertheless falls in love with him. This love extends to a willingness to kill on his behalf, a quality he seeks to exploit as the plot develops. She becomes wary of his friends Ageha and Edogawa, seeing them as unwanted connections to her love interest and plots to remove them from the picture, which may happen as the plot develops (gaining a screen such as that seen on the left). Uuji, seeking his own destruction and finding an attractive quality to her desire to kill Ageha and Edogawa, arranges it so she will kill him as well.
Irisu Syndrome features multiple endings based on the player's progress and actions.
- Bad: If the player loses in Metsu mode six times in a row, Irisu violently murders Ageha, Edogawa, and Uuji in a fit of insanity. She is then seen watching a news report of Edogawa's body being found.
- Good: If the player scores more than 20,000 points, Irisu calms down but then seemingly attacks Ageha (and by extension, the player) in a jump scare. However, it's revealed to be a birthday party for her. The two bond although it's possible that she may harm Ageha as she erases her from a photograph (likely due to Ageha's liking of cats).
- Extra Stinger: If the player acquires 100,000 in normal mode before playing "Metsu" mode, it is shown that Irisu discovers Uuji's picture of a bunny suicide, the image used by Uuji to goad her into the killings in the first place. She sketches a horrific, red-eyed demon rabbit upon it, implying she will go on her rampage and kill the three.
- "Metsu": If the player acquires all journal entries and gains 500,000 points in "Metsu" mode, Irisu is welcomed into the group of friends. Ultimately, this is the best ending given the scenario.
- Irisu is also a bit of an Internet villain, appearing in some Creepypasta and on the Darth Wiki article of the game at TV Tropes.
- It is implied that her red eyes and berserker rage are the result of demonic possession. Specifically, it is by the bunny that Uuji killed prior, seeking revenge for its death and the mistreatment of Irisu.