Kei Misumi works as the school nurse at Seifu High School and is Rin Misumi's older sister, with Rin serving as the head of the disciplinary committee. She moves into the apartment adjacent to Hayato Izumi alongside Rin, facilitating regular encounters with the main characters. Her outwardly harmless or indolent impression masks a distinctly uninhibited and perverse nature, frequently demonstrated through sexually suggestive conduct or revealing attire that starkly opposes Rin's strict demeanor and routinely causes her sister embarrassment.

After Rin uncovers Ui Wakana's underwear in Hayato's apartment, Kei becomes persuaded that Hayato is a crossdresser due to his fabricated story concealing his cohabitation with Ui. She actively "supports" his alleged crossdressing interest through provocative tactics involving inappropriate advances or awkward scenarios, intensifying Hayato's efforts to preserve secrecy. Despite her teasing, Kei knows the truth about Hayato and Ui's living arrangement, having learned it from Ui's parents, but accidentally discloses this secret to Rin, sparking jealousy and conflict.

Kei exhibits a mischievous fascination with romantic dynamics, particularly involving Hayato, Ui, and Rin. She openly taunts Rin about her developing feelings for Hayato, leveraging the situation for entertainment while simultaneously providing Rin emotional backing concerning her romantic challenges. Her character primarily instigates comedic or risqué situations that escalate tension between Hayato and the female leads, maintaining this role consistently throughout the series' official media without significant personal growth or expanded backstory beyond her established traits.