Oshima is a supporting character in the anime Umi Monogatari. She is a teenage girl from the island where the story is set and a schoolmate of Kanon and Kojima. She has brown eyes and brown hair cut to neck length.

Her most prominent trait is her open romantic interest in Kojima, which she makes conspicuously known rather than keeping private. Kojima does not return her feelings, and she views Kanon, Kojima's ex-girlfriend, as her rival for his affection. This places her on the edge of the central emotional drama among the main characters. Her persistent pursuit of Kojima deepens the tension around Kanon's fears about love and rejection, and her presence connects to one of the series' emotional themes: Kanon's worry that Kojima would leave her grew out of her own insecurity, even though he had cared for her all along and never intended to end their relationship.

Oshima's motivation centers on winning Kojima's affection, and her role in the story remains within the everyday school and island life rather than the supernatural conflict involving the priestesses. No special abilities are attributed to her, and her personal development is not a focus of the series. She is nevertheless counted among the few people whose hearts are not seized by Sedna's darkness, which suggests a degree of emotional resilience.