Riku is the adopted brother of Kai and Saya, the youngest of George Miyagusuku's three adopted children. He is a very kind and intelligent boy who loves to cook. His gentle nature even rubs off on the reclusive Hagi to some degree, particularly after he brings Hagi a plate of food and asks him to join the others. Hagi teaches Riku how to play the cello and protects him from a bad injury after Riku falls from a moving train.
After Diva drinks most of his blood at the "Zoo", Saya is forced to turn Riku into a chevalier to save his life. Afterwards, Riku shows a compulsion similar to Hagi's to protect Saya at any cost. Despite a brief estrangement, Riku remains close to his older brother Kai and works to heal the broken relationship between Kai and Saya after Kai learns the truth about her.
Draining Riku's blood seems to cause Diva to develop a strong interest in him. She takes Karl to the Red Shield headquarters to find Riku. Once she finds him, she rapes him and then kills him with her blood. In the light novel adaptation, it's explained that something in Diva's eyes causes the Chevalier blood in Riku to react with a "maddening desire" that causes him to be incapable of escaping or physically resisting Diva's actions.
After Riku's death, Kai has a piece of his crystallized body made into a pendant, which he wears around his neck to remember his lost brother.
