Riku is the adopted brother of Kai and Saya, the youngest among George Miyagusuku's three adopted children. He is a kind and intelligent boy with a passion for cooking. His gentle demeanor even influences the reclusive Hagi to some extent, especially after Riku brings him a plate of food and invites him to join the others. Hagi teaches Riku how to play the cello and saves him from a serious injury when Riku falls off a moving train.
After Diva consumes most of his blood at the "Zoo," Saya has no choice but to turn Riku into a chevalier to save his life. Following this transformation, Riku develops a compulsion similar to Hagi's, to protect Saya at all costs. Despite a brief estrangement, Riku remains close to his older brother Kai and works to mend the fractured relationship between Kai and Saya once Kai learns the truth about her.
Diva's consumption of Riku's blood seems to spark a strong interest in him. She takes Karl to the Red Shield headquarters to locate Riku. Upon finding him, she rapes him and then kills him using her blood. In the light novel adaptation, it is explained that something in Diva's eyes triggers a reaction within Riku's chevalier blood, causing a "maddening desire" that renders him incapable of escaping or physically resisting Diva's actions.
After Riku's death, Kai has a piece of his crystallized body fashioned into a pendant, which he wears around his neck as a memento of his lost brother.
