Born into hardship, Ryo Atsumi shoulders responsibility for her three younger brothers, Ryotaro, Keita, and Yamato, while enduring an alcoholic father's neglect and physical abuse. Driven by necessity, she provides through theft and odd jobs. A past attempt by her father to sell her into prostitution, thwarted only by Ryotaro, fuels her fierce protectiveness toward her siblings and deep resentment.

Initially brash, cynical, and manipulative, Ryo survives through deception. She targets Tamahiko Shima, stealing his wallet and a meaningful bookmark while mocking his disability. Resentment toward Yuzuki Tachibana, stemming from Yuzuki's apparent contentment after being sold into the Shima family, drives Ryo to sabotage their relationship with false accusations of an affair and the destruction of the bookmark.

Sustained interactions spark change. Tamahiko's tutoring of her brothers fosters mutual respect, leading Ryo to gradually shed her defenses. She offers Yuzuki a sincere apology, forging a genuine friendship where Yuzuki sees Ryo as an older sister. Ryo also develops unrequited feelings for Tamahiko but consciously suppresses them, valuing his friendship and respecting his relationship with Yuzuki.

Key events further define her growth. She supports Ryotaro's difficult decision to accept a Tokyo apprenticeship, prioritizing his future over her fear of separation. During the Great Kanto Earthquake, she joins Tamahiko's desperate search for Yuzuki and Ryotaro in the devastated city. Later, she aids Yuzuki against pressure from the Shima family and reveals critical information to Tamahiko about Yuzuki's forced departure.

Ryo's journey concludes as she accepts a marriage proposal from Ryotaro's Tokyo employer, whom she met after the earthquake. She later marries him, becomes pregnant, and attends Tamahiko and Yuzuki's wedding, embodying her transition into a stable future.