Tsukasa Mukai

Mitsumi is a classmate and has been friends with Sōsuke since middle school. He knows about Sōsuke's past and personality and is one of the few friends Sōsuke opens up to. He is sincere and kind-hearted but finds it difficult to talk to girls. He is a member of the Geoscience Club, Astronomy Club, and a basketball interest group.

Although he understands Sōsuke, he secretly thought of him as cold-hearted because he rarely opens up. He knows about the time Mika confessed to Sōsuke.