User blog comment:Xilinoc/Ch. 572 (inb4 "Kenpachi doesn't have Shikai")/@comment-493809-20140313171940

I don't think Guenael is actually a figment of this kid's imagination - I think he's just the type of character that thinks "The whole world is just a figment of my imagination."

Or, as somebody lower down suggested, maybe his ability to manipulate someone is based on their way of thinking (Guenael may be like Schrodinger from Hellsing, who needs to recognize himself to exist).