Ryuushi Shiratori, an 18-year-old aspiring children's picture book artist, moves to Tokyo to attend Emperor's Design School. He takes up residence in the Narutaki-Sou apartment complex, owned by his mother's cousin and managed by his second cousin, Aoba Kozue. His birthday is September 9.

Though he initially doesn't recall their childhood meeting, Kozue remembers. Ryuushi quickly engages with Narutaki-Sou's eccentric residents, including Kozue, whose personalities shift unknowingly under stress. His kind-hearted nature compels him to frequently assist others, often neglecting his academic work and artistic goals as neighbors like Momono Megumi regularly disrupt his studies with impromptu gatherings in his room.

Throughout the series, Ryuushi develops romantic feelings for Kozue and grows protective of her, gradually accepting all her alternate personalities as integral to her identity. His surname "Shiratori" (白鳥, meaning "white bird") ties into the series' theme of color symbolism representing character traits. He occasionally serves as a grounding presence among the quirky inhabitants, yet faces comedic challenges such as being persuaded into cross-dressing by Tamami Chanohata; during one disguise, his inadvertent use of masculine speech confused one of Kozue's personalities.

Ryuushi's character arc focuses on balancing his artistic ambitions with his relationships, particularly his deepening bond with Kozue. In the manga's conclusion, his unwavering affection for every aspect of Kozue helps integrate her personalities into a single identity, leading to their marriage and family.