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.

Spoiler

The Kakugane Tokiko used to revive Kazuki turned out to be Black Kakugane III, which transformed him into a being just like Victor. He is then pursued by the 'Muto-assassination team', as well as his former commander, Captain Bravo. During his assassination, he was dubbed 'Victor III' by the Alchemy Warriors. In the end, he sacrifices himself when he sends himself and Victor to the moon with Sunlight Heart +. In Buso Renkin Period, however, he returns to Earth successfully with help from Victor's Buso Renkin, Fatal Attraction. As soon as he returned to Earth, he and Papillon settled their scores, with Kazuki claiming victory. After this, he returns to a normal life with Tokiko.