A 16-year-old boy who attends a private academy with his sister. After becoming involved in Tokiko Tsumura's fight and subsequently being killed and revived by a Kakugane (which later turns out to be a dark Kakugane), he is told to simply return to his ordinary life, but instead insists upon assisting Tokiko in her battles. Kazuki is very enthusiastic and friendly, and cares deeply for his friends and sister, Mahiro, tending to put himself in danger or difficulty in order to assist or protect others. Although he has an unusual way of showing it, he does have feelings for Tokiko Tsumura.