one who hunts the majesty of the gods
Let's just get this out of the way first: Fuuma's story is fucking depressing.
I mean, let's be real here: the guy spends 8 volumes basically just standing around reacting to the constant drama in Kamui's life and having occasional creepy moments after his dad dies, then as soon as Kamui decides to save the world to protect Fuuma and Kotori, Fuuma gets shafted and has to destroy the world just to pick up the slack. Oh and he gets to brutally murder his own sister. X in general is a pile of angst, but jesus.
Anyway. Seishirou does briefly meet Fuuma before he becomes the other Kamui; Fuuma charges into his illusion in volume 6 to protect Kamui and Kotori, and manages to reverse Seishirou's illusion. It's vague, likely to preserve the twist, but I think Seishirou could likely tell he was the other Kamui; most of the characters seem to be clued in on the plot long before the people actually involved are. (Thanks CLAMP.) Subaru only ever sees Fuuma through Kamui's mind; otherwise, his first meeting with the other Kamui is during their fight in volume 12.
I don't wish to rehash the plot here, particularly because I've already discussed the eye thing at length, but I do want to talk about Fuuma's ability to see wishes and discuss his motives. Because X is unfinished, and because I have no fucking idea how it's going to end, this will dip into speculation at points.
Now, leaving the eye thing aside, Fuuma tells Seishirou in volume 12 that Subaru's true wish isn't what he thinks it is. From volume 16, we know that Seishirou believes that Subaru's wish is to kill him, and that Subaru's true wish is to be killed by Seishirou. Pretty important difference. But why does Fuuma tell him, and why doesn't Seishirou listen?
Well, because he's too set on the idea that Subaru has to kill him, really. I think it would have taken a lot to get it through Seishirou's head that Subaru's wish wasn't to kill him, and in fact it's not until he's bleeding out that Seishirou seems to realize that oh, Subaru is too kind for this anyway:
Seishirou: I should have... seen it. You've never... had it within you... to kill. Your heart... is too kind... for that.
PS, I'm about to tell you I love you and also leave you my eye so you'll become the Sakurazukamori. Have a nice life!
Seriously, what the fuck.
Of far more interest, though, is when Fuuma finds Subaru in volume 17. Given dialogue in volume 16, it seems to be about a month after Subaru's fight with Seishirou, but the X timeline is vague at best so who knows when it is. Anyway, Subaru seems to have found the place where Seishirou killed his mother (as seen in his sidestory at the end of volume 16), including a lovely bunch of off-season flowers.
Fuuma: To be killed by the Sakurazukamori... was your greatest wish, and you here you stand. That said... it's still possible for you to grant what he most desired.
Subaru: Did he tell you what that was?
Fuuma: No. But I still know what he wanted. I can discern any person's greatest wish. Know instantly what burning desire drives them. And in his case, that right eye of yours has the answer.
Hey. Hey Fuuma. You're the one who poked Subaru's eye out in the first place. Just saying.
All joking aside, I talked about this here, but it's worth bringing up again. Seishirou's wish was for Subaru to kill him, of course, but he also desperately wished for Subaru to live. That was incredibly important to him. He may have fucked up Hokuto's spell, but he did ultimately end up fulfilling it by giving Subaru his eye: he gave Subaru something so precious that it would make Subaru protect his own life to defend it, rather than throw his life away. That's important.
Now, here's the part that just makes me want CLAMP to finish the goddamn series already, from the same scene:
Fuuma: I... might end up doing something similar myself.
Subaru: To... Kamui...?
Like. What the fuck. Given the context, Fuuma seems to be saying that he'll end up making Kamui kill him, or that he'll ensure that Kamui wins. A lot of his actions in volume 18 seem to lead to this conclusion, as well. But to be honest, I genuinely have no idea. There's a ton of foreshadowing in X, but I also fully expect CLAMP to pull a plot twist and make things go the completely opposite direction. I have no idea with them anymore.
Not to mention the fact that Subaru actually bothers to ask Fuuma what his wish is. Personally I think the fact that he asks is hugely important, given that Subaru uniquely understands that hey, everyone involved in this fight is involved because they have personal interests at stake, even if those interests end up with you killing the person you're in love with Because Fate. And he knows that Fuuma's only involved because fate screwed him. So the fact that he bothers to ask what Fuuma wants — Fuuma, who has been so royally screwed by the world — yeah, I think that's important. Because even after losing everything, Subaru is kind enough to wonder.
Subaru: So... what is your wish?
Fuuma: It is... something that only Kamui can fulfill.
And of course, what's shown when he says this? Kamui holding the sacred sword. Yeah. About that whole foreshadowing thing.
The last few chapters of X show Subaru at Fuuma's side. Hinoto and Kakyou call him a Dragon of Earth, but so far his role seems to be trying to urge Kamui to urge his real wish. Regardless of which side he's on — regardless of what Fuuma's wish is — I do think that Subaru has realized that Kamui's true wish will decide the fate of the entire world. And right as Kamui seems like he's going to do that...
...yeah, so much for that.