He is a ghost-like entity that possesses all of Banri's memories and personality traits prior to the accident on the bridge where he lost his memories.
As Banri's ghost, he obviously has the same appearance as him, apart from the fact that he wears the same jacket he wore on the bridge the night of the accidents.
As Linda stated, Banri was quite unreliable before he lost his memories; one of the reasons she couldn't leave him alone. Though he was quite weird, his classmates liked him, and were very worried for him after the accident. The most distinguishing trait of Banri's ghost is his love for Linda; a fact that would lead him to curse his current self when the latter disrespected his feelings for Linda and tried to make it like he and Linda were just senpai and kohai at the same university. Banri's spirit, despite being unable to communicate with his present self, thought of him as a little brother, and was desperate to save his life and that of his classmates—Chinami Oka, Kōko Kaga, Mitsuo Yanagisawa and Nijigen—even though he had tried to ruin their plans for a day at the beach. In the end, Banri's Spirit finally got the answer to his confession from Linda—a sincere and passionate "yes"—which ultimately led him to leave the world of the living in peace, leaving to the current Banri his mother's ring so that he could give it to the girl he loved.