Umi Hayama is a second-year student at Takamidzuka High School living in the Tsugumi Dormitory. She has cultivated a reputation for her culinary skills and prepares daily meals for all the dormitory residents. Her appearance is marked by green eyes and blonde hair, which she commonly wears in twintails adorned with ribbons; she stands 1.58 meters tall and weighs 48 kilograms.

Her past is intertwined with that of Wataru Hoshino, her childhood friend. A naturally shy disposition and a strong aversion to solitude meant she spent much of her youth in his company. She lost her mother during her childhood. Some island residents harbor ill will toward Umi and her father, a sentiment rooted in a past intimate relationship between her mother and Wataru's father.

She sustains a longstanding romantic affection for Wataru Hoshino that began in their early years, though he largely remains unaware of her feelings. Her character has evolved from a reserved child into a supportive figure, demonstrated through her steadfast dedication to the dormitory community.