Saku Ezomori is the older brother of Nozomu Ezomori, also a wolf spirit. He first appears in the second volume of the original light novel series and is often seen riding a motorcycle. Saku reveals his love for Chizuru Minamoto. To prevent Kouta Oyamada from becoming intimate with Chizuru, Saku places a magical bracelet on Kouta that inflicts intense pain if Kouta experiences sexual arousal. This bracelet remains for several days. Kouta and Chizuru later fuse into a combined form during a confrontation with Saku. In this fused state, they defeat him, resulting in the bracelet's removal.

Saku possesses white hair and frequently wears a black jacket. His surname, Ezomori, means "obedience," while his given name, Saku, signifies "conjunction" or "first day of month."