Otonari-san

Sasaki lives in an apartment and is a middle school student who lives in the room next to Sasaki's. Neglected by her biological mother, she was often left unattended in front of her house. Feeling sympathy for her, Sasaki began giving her bread and sweets, which led her to develop an abnormal attachment and affection for him, turning her into a yandere. She becomes a participant on the demon's side in a death game between angels and demons as an apostle of Abaddon. Her surname is Kurosu, as depicted in the story, but her first name is unknown.

In the first season of the anime, her tragic circumstances and obsession with Sasaki were hardly touched upon, but this was supplemented in the 13th episode of the comic version, "Neighbor's Perspective," a voice comic video released on the KADOKAWAanime official YouTube channel.

She does not appear in the web version and is a character newly added in the book version. The author decided on a middle school setting as a compromise between the existing heroines, a high school student (Hoshizaki-san) and a magical girl (elementary school student), influenced by the editor's suggestion to consider the gap between them. At the time, age-gap romantic comedies were popular in the light novel world, and death games were trending in the manga world, which influenced her character's role.