Fuyutsuki :

Fuyutsuki is a quiet man. He has his own principles of ethic and moral, but is not strong enough to act according to his principles (which would make him oppose SEELE directly).
He is a figure of authority, but is a decent man in his way.
It is not easy to classify him in the Systema Sephirotica, the only missing Sephira would be Kether ... but Fuyutsuki doesn't fit into Kether's profile. I believe, that Fuyutsuki is a Chesed incarnation like Gendo. I'll try to prove that :
Fuyutsuki is second in command, that gives him authority (briatic correspondence of Chesed). He obey's the orders from SEELE and Gendo (virtue), but unlike Gendo, he isn't self-righteous (illusion), nonetheless he shows humility (virtue, obligtion). I think, that Fuyutsuki and Ikari are two sides of the same coin. Where one achieves humility and fails to rebel against an ideology (Qlippah) he doesn't like, the other is tyrannic, self-righteous but pursues his own goals inside SEELE's schemes.
Now, if Gendo AND Fuyutsuki are Chesed, who is Kether ?
I would say, that Nagisa Kaworu is a Kether incarnation.