Knight Flyer: Tokyo Mahoujin

When Asahi was a young boy, he had a friend named Sakuya who only came out when there was a full moon. One day, she says that she has to leave him, but she gives him a small pendant that she says will protect him in her place. Ten years later, someone with the same name, power and aura as Sakuya saves him, but it's a guy!?

(Source: MU)