Otone Jinnai is the older sister of Enta Jinnai and teaches physical education at Sara Middle School, instructing students Kazuki Yasaka and Toi Kuji. She has long pigtails and light green eyes, typically wearing a lavender shirt under a white jumper. An early design note mentions she previously wore glasses.

She maintains a stable, caring relationship with Enta despite occasional bickering. She expresses concern over his spending habits and becomes visibly upset when he is injured, comforting Kazuki while concealing her own distress. Her protective nature includes reminding Enta about financial responsibility.

Otone is romantically involved with Chikai Kuji, Toi's older brother, but loses interest after the kappa prince Keppi assimilates his shirikodama—a metaphysical manifestation of desires. Before this, she showed strong affection for Chikai, including a desire to kiss him. Documented connections include friendship bonds with Kazuki (OtoKazu) and Toi (OtoToi), plus a femslash dynamic with Sara Azuma.

Professionally, she introduces transfer students like Toi to their class. She shows surprise when Enta claims friendship with Toi after a package mix-up, indicating awareness of her brother's social ties. During events like Enta's hospitalization, she directly engages main characters, absolving Kazuki of guilt and blaming the actual perpetrator.

Background notes reveal early scripts conceptualized her as a middle school student instead of a teacher. Her design bears resemblance to Lulu Yurigasaki from *Yurikuma Arashi*, another Kunihiko Ikuhara series. Her given name Otone combines "sound" (音) and "tranquil" (寧), while her surname Jinnai means "team" (陣) and "inside" (内).