Itsuki was raised in the Bahbem Institute with Makoto Isshiki and Helena Bahbem, and is the adoptive brother of Quon to whom he is very attached to. He is the leading researcher of the RahXephon and Dolems and studies their relationship with music.

He once dated Haruka and is aware of the special connection between her and Ayato. He once tried to pilot the Rahxephon at Makoto's urging only to have the water surrounding it slam him against a wall, taking the form of Reika afterwards. For that he is highly envious of Ayato.