Ai to ka Koi to ka, Kudaranai.

Kawai Yuuma considers Kuramoto Suzuka, a year younger and the sister of his friend Akira, as a little sister. Their dynamic shifts when Akira begins a relationship with a senior coworker, prompting Suzuka to express confusion about romantic emotions. Yuuma, who has his own uncertainties about love, becomes intrigued by the idea. Despite knowing it's inappropriate, he acts on his curiosity, leading to a complicated arrangement between them. They agree to keep things going only until they find someone they genuinely love.