Isako Toriumi is a teacher at Gekkoukan High School and the homeroom teacher of the main character's class in Persona 3 and Persona 3 FES, as well as the teacher for both the male and female protagonists in Persona 3 Portable. Other playable characters in her homeroom include Yukari Takeba, Junpei Iori, and eventually Aigis. She takes her teaching seriously and is adept at handling her students, but also has a hidden side filled with stress and the frustrations of the real world.
She is voiced by Yuka Komatsu in the Japanese version of the game.
Ms. Toriumi frequently plays the MMORPG "Innocent Sin Online" as a means to relieve her stress from work. Unknown at first to both the male protagonist and to the player, it is within the context of Innocent Sin Online that she appears as the player Maya. As Maya, she is the first person to greet and befriend the main character; she also suggests that he give his online avatar the name Tatsuya. Maya forms a connection with Tatsuya, and is very happy when he puts up with her work-related rants, allowing her to blow off some steam.
Over time, Maya reveals more and more about herself. She says that she is a teacher, and then reveals that she has a crush on one of the students in her class. When the name of the student she has a crush on is revealed to be that of the main character, it is revealed that Ms. Toriumi is Maya. However, she does not realize that Tatsuya is in fact played by him.
Eventually, Ms. Toriumi comes to the realization that she needs to stop hiding from the world outside of the MMORPG. She logs on to be with Tatsuya one last time to say goodbye and thanks him for being such a good friend to her. She leaves him a parting gift in the form of a message written in sand within the game.
In FES, the player can accidentally reveal that he is Tatsuya to Ms. Toriumi, who upon learning this is deeply flustered and embarrassed but will still try and ask him out for dinner.
Ms. Toriumi returns in Persona 3 Portable and once again serves as the male main character's Hermit Social Link. However, she does not fill this role in the female protagonist's side of the game. The Hermit Social Link on the girl's side is Gekkoukan High School student Saori Hasegawa.
If the player manages to max out the Hermit Social Link, the player can fuse the persona Arahabaki in the Velvet Room.