Umi Matsuzaki, a disciplined 16-year-old in 1960s Yokohama, balances school life with managing Coquelicot Manor, her grandmother’s boarding house. She oversees chores, cooks for boarders—medical student Miki Hokuto and artist Sachiko Hirokouji—and cares for younger siblings Sora and Riku during her mother’s medical studies abroad. Her father, naval captain Yūichirō Sawamura, perished in the Korean War after his ship struck a mine. Umi honors his memory daily by raising signal flags at dawn and dusk, a ritual symbolizing "safe voyages" for ships, and placing fresh flowers with a glass of water before his photograph.

Her structured life shifts when she joins efforts to save her school’s Latin Quarter, a decaying clubhouse slated for demolition. Inspired by journalism club member Shun Kazama’s bold protest stunt, she spearheads the building’s cleanup, earning the moniker "goddess of good luck." Their collaboration deepens after Umi shares a photo of her father with two comrades, sparking Shun’s suspicion they share a biological parent. Suppressing romantic feelings under presumed siblinghood, their tension crescendos in a mutual confession. Veteran Yoshio Onodera later clarifies Shun’s adoption by Yūichirō following his biological parents’ deaths, resolving the conflict and allowing their relationship to flourish.

Umi’s shoulder-length dark brown hair is often styled in twin braids, paired with her second-year uniform: a white sailor shirt with blue trim, dark blue skirt, and red ribbon. Casual outfits feature a light pink blouse with a hot pink skirt and sweater. Her name ("sea") mirrors her coastal roots, complemented by siblings Sora ("sky") and Riku ("land"). While the original manga surnames her Komatsuzaki—incorporating "pine tree"—the film omits this detail. Her narrative arc embodies harmonizing tradition with progress, honoring her father’s legacy while actively reshaping her community’s future.