Otonari-san is a junior high school girl living in the apartment next door to Sasaki. She becomes the Apostle of the demon Abaddon, getting involved into the Death Game between Demons and Angels.

Her parents divorced during her early childhood, and she suffers neglect from her biological mother. Otonari suffered constant physical and psychological abuse, being harmed, verbally assaulted, and even denied basic necessities such as food. She given up on further education, leaving her studies in shambles and spent most of her time collecting leftover school lunches for food. Sasaki, feeling sympathy for her abandoned state outside the house, would give her bread and sweets. Sasaki never brought her into his home, knowing that bringing a minor inside could constitute confinement, even with consent. Sasaki had reported her mother to child welfare services, but the mother was skilled at covering things up, and attempts to secure her protection failed.

When she was nearly raped by her mother's boyfriend, she was chosen by the demon Abaddon to participate in a Death Game between Angels and Demons, becoming Abaddon's Apostle.

Using rewards earned by defeating enemies in the Death Game, she immediately asked Abaddon to manipulate Sasaki's sexual desires to make him attack her, nearly staging a false rape incident. However, Sasaki's grit and recovery magic thwarted her, and since the reason for her failure remained unclear, she gave up on that approach afterward. Later, while maintaining a magical barrier, Sasaki was swallowed into the isolated space serving as the Death Game battlefield. This revealed the existence of the Death Game to Sasaki and Shizuka Futari.

After Otonari lost her mother when the apartment was destroyed by a bomb, she was taken in by Shizuka.

Personality: At first glance, she appears to be a quiet girl, but deep inside, she harbors pent-up passions and the murky mess of her living conditions, becoming a yandere who developed an abnormal obsession and affection for Sasaki. She deluded herself into believing that Sasaki was lonely like her, making them a perfect match. She came to think that dying together after loving each other was their happiness. She schemed to get him to attack her, exploit his guilt, and cross the line to commit double suicide. She couldn't accept Sasaki sacrificing himself for her sake, aiming instead for mutual consent to die together as the culmination of their love.

She is serious, stoic, apatethic and ruthless. She feels nothing about killing enemies one after another shortly after joining the Death Game.

Abilities: Through her contract with Abaddon, Otonari gained the ability to drain life energy throught physical contact, easily rendering humans unconscious and even other powerful beings like Peeps or the Mgical Girls. The energy Otonari absorbs becomes her own nourishment.