Kusanagi on Fuuma: "The person i know, rather than to say that he is like me, he is like anyone, and yet no one. it's as if he has given up himself and plays that "someone" the others wish him to. but i don't think that's really a good thing."
(translation taken from One of the best online translation sites I've found
That explains quite a bit about Fuuma. He always seems to want to grant wishes.. which is why he took Subaru's right eye, and why he seems to look like someone else to everyone else.
Nataku-> Kazuki's father
Yuzuriha-> Kusanagi (I had to go searching for this one, b/c I couldn't remember if they'd actually met or not. They did.. breifly in vol. 12, when she was running to meet Kusanagi& bumped into Fuuma)
In any case... As of vol. 18, I still don't think Kamui can make his own kekkai yet.. but Fuuma says that if Kamui learns how to make a kekkai, Fuuma will also gain a new power and get stronger. (Which would make him even more of an evil bastard.)