Fuuma and Kakyou start out with a strictly business-like relationship: Kakyou can help Fuuma, and in return Fuuma will grant his wish. At this point there's no sign at all that either one sees the other as anything more than a means to an end. Then suddenly we're several volumes along and the situation has completely changed: Fuuma is both VERY possesive of Kakyou and very tender with him (in fact, there is no other character in the series that Fuuma treats with as much tenderness as Kakyou, not even Nataku); and Kakyou accepts this from him.
Some people think that Fuuma's behavior towards Kakyou is manipulative, but since that's highly unlikely unless you also believe that Fuuma is evil, which I don't, so I personally discard that idea right away.
There's obviously the possibility that Fuuma's behavior towards Kakyou is a response to his drive to be what other people need or want him to be. That's an interesting idea, and I don't think it necesarily means that Fuuma's actions towards Kakyou aren't genuine, or that they aren't something he has control over. I think the key here is that Fuuma grants wishes because he chooses to (or because he accepts the role, either way it's a conscious decision), because, like he said, it's an expression of his own wish. This can be applied to his behavior towards Kakyou in the sense that he knows Kakyou needs comfort (and someone who dominates him =9) and this need is something that speaks to him and makes him want to give it to him. And of course, even if he's providing comfort as a response to Kakyou's desires, there's no particular way to be as touchy feely about it as he is. ^_^
The thing that makes this pairing so fascinating to me is the question of Kakyou's wish and how Fuuma will go about granting it (this is also how Hokuto comes in). Here's the thing: although Fuuma has said he's the only one that can kill Kakyou, and although he has made references to Kakyou finally being able to die when The Future plays out, he doesn't say that he will kill Kakyou (I went through the manga and the text translations I have once specifically searching for any instances of Fuuma actually saying "I will kill you" to Kakyo and couldn't find any), all he says is "I will grant your wish". That's a very telling distinction.
Now, if we take what Kakyou says his wish is at face value, then Fuuma will kill him because that is his wish. However! XD If there's one thing we've learned from Fuuma is that people's wishes are rarely what they say are, they're something deeper, something that is hidden (I love that about him BTW, he's like the genies who end up screwing over their owners on technicalities... "be careful what you wish for" indeed). So I don't believe for a minute that Kakyou's wish really is literally to die.
There's a scene in volume 14 that I love: Kakyou is angsting over Hokuto, and Fuuma comforts him and grabs his hand and says "Come on". When Kakyou asks where, Fuuma replies "To the sea. You made a promise with Hokuto to go." Kakyou asks if he knows how to get there, and Fuuma says yes, then Kakyou asks why Fuuma is doing this and Fuuma says "I told you. I will grant your wish".
If Kakyou's wish is what he says it is, to die, then what in the HELL does all this have to do with it? To me the answer is simply that Kakyou's true wish isn't to die, but something else. However, he THINKS his wish is to die. As I see it, he thinks this is his wish now that his original wish can't come true.
This is where Hokuto comes in. When they met in Kakyou's dreamscape, Kakyou told her "I've wanted to meet someone". Kakyou had spent his entire life in an ivory tower, locked in his dreamscape, having no contact with anyone. In other words, Kakyou was utterly alone, and he wanted to meet someone so he could stop being alone and through them, break out of his prison. I think THAT is his real wish is one of two things (but actually probably both): to not be alone anymore, and to go "outside", in other words to be free... the thing is that the reason he wants to go "out" is so he can stop being alone, so really, it all boils down to not being alone.
At any rate, Hokuto. She chides him for implying that "anyone would do" and says you're not supposed to say that to a cute girl, but I think that that's precisely the point, Kakyou wanted someone, anyone, and she happened to come along and was kind to him, because that's just the sort of person she is (see her story with the foreign girl in TB) and thus, he falls in love with her, because she represents what he wants: the end of his loneliness and the way "out". And so when she dies, and he's hurt trying to stop it, he thinks that's it, his beloved is dead and his one chance at happiness is gone forever, so what's the point? It's easier to just give up and stay in his coma and wait for death. And so, since he now believes there is NO ONE ELSE who can possibly be what Hokuto could have been, he comes to believe that the only thing he wants now is death. But what if he COULD have what he originally wanted?
The thing is that just because she did WAS the person that could give him his wish at one point in time, there's no reason to think that Hokuto was his one and only chance to get what wanted. It wasn't that just anyone would have done, it had to be someone willing to be kind, to ease his loneliness and encourage him to "go out" and be free. In fact, in the present (X's present ^^) there is someone that is kind to Kakyou and keeps him from being alone and even offers to take him to the sea (I think that for Kayou the sea is a metaphor for freedom). And that person is Fuuma.
Fuuma CAN give Kakyou what he wants, he can grant his wish and has in fact said he'd grant his wish. That's what it boils down to to me. Of course, there's the problem of Kakyou realizing this, because he clearly doesn't at this point, and I don't think Fuuma can grant his wish for him if he doesn't realize what it is. And of course there is also the small matter of Fuuma's impeding duel to the death with Kamui... My personal, very shippy solution to the entire mess is that if Kakyou would realize it, then Fuuma would HAVE to live and let go of everything else in order to grant his wish and it would all be resolved and they could have lots of hawt secks on a magical beach where the sand doesn't get in uncomfortable places.