He is one of the chosen knights, protectors of the diamond heart. Mamoru was the first one to meet Himeno and the first one to become a friend of her. He is responsible and takes his job seriously and he also has feelings for Himeno. He is the heir to the Mitsuba family (a family well known for having power in the global economy) and the first one to accept his duties as a Knight. Attends school in France and is bilingual in English and Japanese. Has a younger sister named Kyouka.
He is Himeno's first love.