source: wikipedia
Maya met Tatsuya Suou's group long before the Joker incidents began in Innocent Sin. Following the demise of her father back when she was in junior high 10 years ago, she prayed in Araya Shrine all the time, and in one instance ran into some kids (Tatsuya, Jun, Lisa, and Eikichi) who called themselves the Masquerade, and were role-playing as Phoenix Ranger Featherman R, their favorite TV sentai heroes. She came to play with them every day, and taught them a game called "Persona" which was believed to show you what you would be like when you grow up.
Maya became particularly close to Tatsuya and Jun, who she considered as doppelgangers, or two sides of the same coin. The two boys made a pact to protect their "big sis" as per their role as superheroes.
The time came when Maya's mother decided to move out of town. Maya bid goodbye to her friends, but she was locked in the shrine by three of the kids, determined to stop her from leaving. Tatsuya was later detained in the same place when he started to protest about how badly they were treating "big sis". A fire soon engulfs the shrine, started by a troubled youth named Tatsuya Sudou, son of the foreign minister. Maya helps Tatsuya escape, but Sudou stabs him. She awakens to her Persona and manages to break the lock despite a terrible burn she sustained at the right side of her chest. Maya is never seen again and was presumed dead, starting the rumor that the ghost of a young girl appears, haunting Mt Iwato at night.
In reality, Maya has just moved away and did not return until years later, taking a job in Aoba as editor of Kismet Publishing's teen magazine "Coolest". She meets Tatsuya's group again in Seven Sisters High School while gathering information on the Joker Charm, accompanied by her partner Yukino Mayuzumi.
Her infectious cheerfulness and positive outlook on life becomes the very backbone of the group, getting them out of many dire situations they encounter during the course of finding out the true purpose of the Joker. So upon her demise at the end of the game at the hands of Nyarlathotep's minion, they could not immediately accept Maya's dying wish for them to forget about her quickly so they would not carry a heavy burden in their hearts.