Itsuki Kisaragi  

, 如月 樹

Itsuki grew up alongside Makoto Isshiki and Helena Bahbem in the Bahbem Institute. He considers Quon his adoptive brother and has a strong bond with him. As the leading researcher on RahXephon and Dolems, he explores their connection to music. At one point, he was romantically involved with Haruka and is aware of her unique relationship with Ayato. Despite this, Itsuki feels envious of Ayato due to an incident where he attempted to pilot the Rahxephon at Makoto's urging but ended up being slammed against a wall by water surrounding it, resulting in him taking on the form of Reika.