Aiko Honjou
About
Aiko is the spirit responsible for all the deaths in and around the cursed apartment. She refers to Yuko as her "big sister" and can flip between sweet and murderous in an instant.
Aiko used to live at Mejiro Villa with her mother, Hikiko, and was bullied at school for being raised by a single mother. She was very close with her grandmother, Asako, and sometimes hung out with the landlord's son, Kanetsugu. Aiko was declared missing at age 7 after a fire consumed Mejiro Villa, where the apartment complex that houses the cursed apartment was later built.
After the death of her grandmother, Aiko became the victim of sexual abuse at the hands of Kanetsugu (who also taught her how to use parasitic spiritual powers), and physical abuse at the hands of her mother. The abuse culminated in Aiko and Hikiko's deaths, which Kanetsugu covered up through the fire; while Ichinose theorizes that Hikiko was the one who killed Aiko, it was actually Kanetsugu who sexually abused and killed both Aiko and Hikiko. Aiko's body is buried in Kanetsugu's backyard, with a tree planted on top.
After merging with Yuko, Aiko unlocks memories of Yuko's own abuse at the hands of Kanetsugu, and her having been raised and abused by him to be a vessel for Aiko's spirit. The two then work together to get revenge on him. Later, it is stated that Yuko's mummified corpse was found in a psychiatric clinic, despite several claiming that they had met and spoken with her since her supposed death. It is implied that Yuko and Aiko are still wandering the earth as a merged entity, with some being able to see them.