All in a name
Kanji analysis
First of all, a disclaimer: I'm perfectly aware that Japanese names don't really literally mean what their kanji break up into, and that mostly they're used for the sound, not the meaning. That being said, this is CLAMP and X we're talking about, and there's several instances where they specifically talk about the meaning of certain names ("Kamui" and "Magami", for instance), so it isn't inconceivable that they may have had an ulterior motive in picking out more of the names of their characters... and this is particularly likely when you realize what "Fuuma" means.
At any rate, this is Fuuma's name in kanji:
These are the individual meanings of each kanji in Fuuma's name:
- momo; peach.
- means birth, life (shou, sei); has several other meanings, most having to do with the ones already discussed: to give birth, childbirth, to live. It also means pure or raw, among other things.
- fuu; seal. If the kanji is used at the end of the word (-ippu), it means sealed letter, sealed document, enclosure. Incidently, the word fuuin (seal) in nanatsu no fuuin ("the seven seals") is composed of this kanji plus another one. Coincidence that "Seal" and "Fuuma" have something in common? I wonder... 
- ma means just, right, due (east), pure, genuine, true; shin is truth or reality.
Monou
At first glance, it would seem that there really is not much room for possible hidden meanings in Monou; afterall, "peach" and "life" don't really make a lot of sense together. Until you start looking into the symbolism of peaches in Japan, and you find out that peaches, because they are linked to the coming of spring, symbolize new beginnings, marriage, and, most importantly in this case life, longevity and inmortality (there is a Chinese legend about the peach tree of the Gods, which blossoms every 3 thousand years and whose fruit, which grant inmortality, take another 3 thousand to ripen). The fact that peaches symbolize life becomes particularly interesting when you consider that the second kanji in "Monou" literally means life.
Of course, having a family name that means "life" seems either totally random or ironic when 3 out of 4 members of the family are dead, in fact, died violently while still relatively young. Unless you consider what the Monou family is in X. For one thing, they are very much what gives birth, so to speak, to the story; yes, it's Kamui that decides the fate of the world, but over and over, it is the Monou family that is the catalyst for events in the story, starting with Monou Saya's death, which sets everything in motion.
Another way to look at it is what the Monou family means to Kamui. While he and his mother lived near the Monous in Tokyo, Kamui was happy. Years later, when everything is falling apart, he idolizes the time with the Monou family as a sort of utopia, a time full of happiness, before death entered the picture and changed everything. So it really doesn't seem so strange to me to think that CLAMP may have specifically picked "Monou" for this family.
(And, specifically for Fuuma, I find it amusing that also appears in Kinomoto Touya's name).
Fuuma
And now for the really interesting part...
Although this is of course a matter of interpretation, the meaning of the kanji in "Fuuma" () are particularly interesting. A popular interpretation of them is sealed truth. This meaning can be applied specifically to Fuuma and his role in several ways. The most inmediately obvious is fated role as Kamui's twin star, in the sense that the truth about Fuuma as something that almost no one, not even himself, knew (his father knew all along, Kakyou saw it in the dream, and Kanoe figured it out through spying in Hinoto's dream). In the sense that Fuuma's destined role was known only to a few, it was very much a sealed truth, like his name implies.
However, there are several other ways in which sealed truth applies to Fuuma. For instance, there is the question of Fuuma's true motivations, specifically of his true wish. Although we've had several important hints about what his wish is, it remains a secret that no one but he seems to be privy to... another sealed truth, and a particularly important one, given how crucial it will be (along with Kamui's wish) for the Earth's fate.
For that matter, Fuuma also knows X's ultimate sealed truth: he knows what Kamui's true wish is, something Kamui himself does not know. And while a few others (Kakyou and Subaru, for instance) know that Kamui's wish isn't what Kamui thinks it is, it seems that Fuuma is the only one that knows what it really is. Still on the subject of wishes, Fuuma has the ability to see the true wish of anyone he meets, and so he can know what others would keep hidden from everyone else (the best example of this is Subaru's wish to die at Seishirou's hands, something no one but Subaru, not even Seishirou, was aware of).
Another interpretation of the kanji, one that not many people seem to stumble upon (and I don't really undertand why) is true seal. One way to interpret it is that Fuuma is, in a way, a seal on Kamui's consciousness and purpose, in the sense that it's been said time and time again, by Fuuma and Kakyou, that Kamui will never win unless he lets go of his supposed wish: to get "the old Fuuma" back; basically, Kamui has to let go of Fuuma, and of the relationship they had, in order tobe able to move forward and do what he has to do. In this way, Fuuma is a seal on Kamui's powers and fate.
My own personal (and somewhat crazy, I admit) theory is that maybe Fuuma literally means True Seal... this is based on several things, such as the fact that it is Fuuma that has the "good" angel wings while Kamui has demon wings. Basically I wonder if maybe Kamui wasn't meant to choose the Dragons of Earth, in which case it would have been Fuuma with the Seven Seals. This is very debateable of course, and it IS just a theory. Or maybe what it means is that Kamui, when he finally knows his true wish, might switch sides (I've always thought that it might ended up being similar to Ashura's awakening as Dark Ashura in RG Veda), in which case, Fuuma would now be a seal. Who knows, really, but it's certainly fun to speculate. ^_^