Momomi Kiyashiki is a St. Spica Academy student council member who maintains a romantic and sexual relationship with fellow councilor Kaname Kenjō. She often exhibits more aggression during their intimate moments while constantly seeking Kaname's affectionate attention. Their relationship features shared activities like bathing together in a clawfoot tub—where Momomi initiates physical contact—and sleeping in the same bed, sometimes with her hand resting on Kaname's breast. Momomi demonstrates fierce loyalty to Kaname's goals, including interference in Amane Ōtori and Hikari Konohana's relationship to benefit Spica in Etoile elections, though she typically avoids direct involvement, preferring observation.

Her snobbish personality manifests through beliefs in Spica's superiority over other Astraea Hill schools. During council meetings, she consistently aligns with Kaname's suggestions and advises president Shion Tōmori, intensifying Spica's internal conflicts around Amane's Etoile candidacy. Momomi actively executes Kaname's schemes, such as encouraging Kaname to seduce Hikari by emphasizing her charisma, without showing overt jealousy. She employs physical affection to manage Kaname's emotions: kissing her in the bath to soothe rage, embracing her admiringly behind curtains during a Carmen performance, and comforting her with rooftop hugs during election anxieties.

A pivotal moment occurs when Momomi discovers Kaname's longstanding romantic fixation on Amane, revealing Kaname's lack of reciprocal love. This prompts Momomi to slap Kaname and leave heartbroken. Their relationship later reconciles when Kaname moves past her Amane obsession, implicitly restarting their bond.

Physically, Momomi has long light brown hair styled in half pigtails with the rest flowing freely, matching light brown eyes. She wears St. Spica's standard uniform and a light lemon-colored nightgown for sleep. She appears in Episodes 7 (collaborating with Kaname on the Hikari scheme), 13 (bath scene and intimate moment), and 15 (shared bath during cultural festival preparations).