Twin stars: the two "Kamui"
Kamui is X's reluctant instrument of fate and its protagonist. He comes back to Tokyo after a seven year absence following his mother's dying request, but upon arriving has no interest at all in submitting to the destiny that is revealed to him. In fact, he spends the first volumes of the manga being a bratty, sullen boy, WAY too sure of his own power but with no wish to control it.
You just KNOW a character like that is in for a rude awakening. XD
Kamui is told that the fate of the world rests in his hands and that he must choose whether to save it or destroy it. In typically teenage fashion, he doesn't want to. Love of his childhood friends Fuuma and Kotori eventually moves him to realize that he "wants to protect a world where Fuuma and Kotori can be happy" and thus chooses the Dragons of Heaven.
Little did he know, however, that Monou Fuuma was actually his twin star, and fated to fill the spot left vacant once he chose a side (in other words, Fuuma became the kamui o the Dragons of Earth). Fuuma proceeds to blow up the surroundings, impale Kamui with convenient (and pointy) debris, and finally kills his own sister. Kamui "goes within" because he doesn't want to face what has happened, until Sumeragi Subaru convinces him to return out of himself. After this, Kamui decides that his wish, the only thing he has left, is to bring back "the old Fuuma".
Fuuma was Kamui's neighbour and childhood friend. At the beginning of the story, Fuuma is a serious young man, apparently the perfect son, brother and student. His grave manner is most likely due to his mother's violent death seven years ago and the consequences to his family. Seriousness aside, he is caring and protective, and eventually manages to regain Kamui's friendship and trust.
One of the great tragedies of X is that, despite his childhood promise to always protect Kamui (or perhaps because of it? promises and wishes are tricky things in the X-verse...), Fuuma is in reality Kamui's twin star, his opposite. When Kamui finally chooses to become a Dragon of Heaven, Fuuma inmediately becomes the Dark Kamui, one of the Dragons of Earth. His first act upon his awakening is to impale Kamui to a wall with the Shinken and shards of glass and metal. Then, before Kamui's very eyes, Fuuma crucifies his own sister and plunges the Shinken into her heart.
He spends the next few volumes basically wreaking havoc on Tokyo by causing earthquakes, and occasionally indulging in what is often described as "molesting Kamui". Fuuma ALSO has the ability to see people's wishes, and in some cases grant them. The phrase "be careful what you wish for" was never more true, since the manner in which he grants wishes often results in the wisher's death. At this point it's very easy to write Fuuma off as simply the "bad guy" of the story. This is not necesarily the case. Quite aside from the role he must play, Fuuma is an infinitely complex character. He is capable of tenderness as well as of ruthlessness, and his ultimate motive is not to kill Kamui but to make him realize what his true wish is. Fuuma's own wish remains a mystery save for one clue: it is something only Kamui can grant.
I should point out here that out of all the ficitonal characters I have come to love in my life (and there have been many), Fuuma is far and away my favorite. Which means that I have many many theories regarding him and his role in the manga. Too many for this site. If you'd like to read about them, check out As You Wish, monou.net's Fuuma shrine.