Kana Sumiya is Sora Aoi's classmate and Nami Aoi's best friend. She and Nami form the entire membership of their school's cosplay club, where Kana designs and creates female outfits. Despite Sora's reluctance, Nami forces these costumes onto him, with Kana providing the garments.

Kana harbors unspoken romantic feelings for Sora, her shyness preventing any direct confession and complicating their interactions. She remains unaware that Nami holds deeper romantic feelings for her, even as Nami actively tries to set Kana up with Sora.

Their relationship transforms after Kana discovers Nami and Sora engaged in sexual activity, leading to a three-way encounter involving all three. During this encounter, Nami's hidden feelings for Kana emerge, and Kana reciprocates by initiating a kiss with Nami. This marks a shift in their dynamic, and they begin dating afterward.

An earlier kiss between Kana and Nami in the series stemmed from Kana's desire to indirectly experience a connection with Sora through Nami, who had shared her first kiss with him. This act foreshadowed the later complexity in their relationships.