**Ritsu Kawai** is the daughter of the legal owner of Kawai Complex, managed by her aunt Sumiko while her mother works abroad in America; her parents are divorced. Initially highly introverted and anti-social, she frequently retreats to the library, becoming so absorbed in reading she ignores her surroundings. Books act as her primary confidants, reflecting her difficulty forming deep human connections. Despite her quiet nature, she demonstrates assertiveness when residents cross boundaries, leveraging her family's ownership to threaten rent increases for control. She possesses a tendency toward Brutal Honesty, often expressing thoughts bluntly without considering their impact, sometimes hurting others. She harbors a secret appreciation for traditional Japanese clothing, noted as a personal fetish.

Socially awkward and lacking interpersonal skills, she struggles to articulate thoughts during conversations, leading to rambling or ineffective apologies. Her emotional range appears limited initially, typically maintaining a blank expression except when deeply moved, amused, or embarrassed; her rare smiles become notable in these moments.

Her development starts when Kazunari Usa moves into Kawai Complex. Despite reticence, she gradually forms a reliance on him, though misinterprets his kindness as pity rather than genuine affection. Their relationship evolves through misunderstandings, including her mistaken belief Usa harbors feelings for a classmate. Interactions with Usa catalyze significant growth. She begins activities beyond reading, like playing video games, and becomes more receptive to friendships outside her comfort zone. A pivotal moment occurs when her former middle school classmate, Hayashi, helps her recognize and accept her romantic feelings for Usa, leading to them becoming a couple toward the end of his second year of high school. This period marks her transition from isolation to a wider social circle, showing increased emotional openness, warmth, and protective instincts for fellow residents, including advocating for them against exploitation.

Additional details: she fears centipedes and dislikes horror movies (contrasting with Mayumi Nishikino). Chinatsu refers to her as "Loner-senpai." Though disliking casual book discussions, she enjoys recommending literature to those with shared interests. Her room, at the dormitory's farthest end, houses an extensive personal book collection. Her name "Ritsu" translates to "rhythm" and "law," while "Kawai" combines "river" and "to fit." Anime adaptations depict her with brown hair and purple eyes, differing from the manga's black hair. Her journey encapsulates a shift from isolation to connection, underscored by evolving relationships and a gradual embrace of emotional vulnerability.