Riku Miyagusuku

Riku Miyagusuku is the adopted brother of Kai Miyagusuku and Saya,  the youngest of George Miyagusuku's t hree adopted children and Saya's second chevalier. He is also the brother-in-law of Mao Jahana Miyagusuku and the uncle of Sakura Miyagusuku and Riku Miyagusuku Jr.

Life In Okinawa
Riku leads a normal, cheerful life in Okinawa with his adopted father, brother and adopted sister. After their father's death, he accompanies Saya on her journey to slay chiropterans.

In Vietnam

Riku accompanies his brother and sister to Vietnam. With Saya gone to the Lycee, he finds himself passing time with Kai. He wanders around in the neighborhood, meets other children, plays with them. There he meets a girl around his age called Mui  who can also hear chiropterans like he does. He and Kai save her once, stay in the orphanage where she stays and get kidnapped. After they break out of the prison and reunite with Saya, Riku's friend transforms and is taken away to cure her.

At The Zoo

Riku goes to the Zoo with the other Red Shield members. He is left alone as others spread out to search for Saya. Then he hears Diva singing and getting drawn by the voice, he soon reaches the tower where Diva was imprisoned long ago. When he reaches the dark cell, he sees a girl and mistakes her for Saya. As soon as Diva's eyes glow bright blue, he realizes that she's someone else. But he is too late to escape Diva who starts draining his blood as he lets out a scream.

Becoming A Chevalier

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.

Assumed Death

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. But once she hound him and tried to rape him, Riku spat Diva in the eye, which gave him enough time to escape, but when the Red Shield Headquarter's self destruct mode was activated, all the exits that Riku can reach are blocked by pieces of the ship. After that, Riku presumably died when he sinks with the Red Shield's Headquarters after it blew up.