Itsuki Kisaragi is an urbane doctor with TERRA who studies music and is part of the analysis team studying the RahXephon. He is also the adoptive brother of Quon. Although most people do not know, he is actually a Mulian and Ayato's twin brother, and his adoptive "sister's" biological son. Like Megumi, he feels inferior to his sibling and struggles with his own worth throughout the series. He is unable to pilot the RahXephon, and once dated Haruka, who now rejects him. He was raised in the Bähbem Institute with Makoto Isshiki and Helena Bähbem. Like Ayato and Quon, he bears the marks of a potential instrumentalist on his stomach.
In other versionsIn the movie, his character is reduced to being a love-sick guardian of the sleeping Quon.
In the manga, Itsuki and Haruka are friends and colleagues, and they eventually marry. On their wedding day, it is revealed that Itsuki is a reincarnation of Ayato.