Fuuma and Kamui are childhood friends who both have tragic fates. Fuuma was born to be Kamui's twin star. The two first meet on a rainy day when Fuuma sees Kamui crouching in front of his house, covering a wet puppy from the rain. The two become quick friends before their mothers even introduce them to each other.
To talk about Kamui, at least in any meaningful sense, is also to talk about Fuuma, who is, after all, also Kamui. This creates problems since there are few characters in the series that are more mysterious, opaque, and just generally frustrating as Fuuma. To speak of his character is to speak of theories and possibilities (at least in the manga), though there is hope that the finale of the series, whenever it should occur, will shed some insight on this terribly complicated character.
In the beginning of the story, Fuuma is the pillar of his family--helping his father and looking after his beloved younger sister. Well liked and respected by those who know him, Fuuma appears to be a solid and trustworthy guy; one who is perhaps not overtly friendly, but certainly kind and helpful. His most important role seems to be that of a big brother to Kotori, who relies on him constantly due to her fragile health. He apparently loves his sister and wants her to be happy.
He is also Kamui's best friend. The two of them met as children and immediately became close friends. As Princess Hinoto later observes, while Kamui loved Kotori the most, he trusted Fuuma more than anyone else in the world. Kamui had no other friends and thus these two people became the center of his world. That, it could be said, is his tragedy.
In some ways, Fuuma was very different from Kamui. Fuuma was raised in a family that was stable, loving and generally "typical", or at least seemed that way on the surface. When the family image began to shatter, it was marked by the death of his mother, leaving him with a father and devoted but fragile sister. Kamui, on the other hand, never knew his father and suffered as an outcast because of this. The only person he had growing up, besides his years with the Monou family, was his mother. Fuuma was well respected in the community and a good son, while Kamui was an exile of sorts, particularly in the anime. As young children, Fuuma seemed confident and affable, while Kamui seemed somewhat timid and melancholy. As teenagers, Fuuma seemed emotionally stable while Kamui seemed volatile. If anybody seems likely to decide to want to destroy the world, at least on paper, Kamui would be a more likely candidate.
When Kamui returns to Tokyo, both Fuuma and Kotori worry about their childhood friend and wonder why he has withdrawn from them. Kotori believes that Kamui is still the sweet person she loved as a child, but it is Fuuma who understands his motivations. He sees his friend's suffering and, though he does not understand the circumstances, wants to support him through his troubles.
Unfortunately, Fuuma has moments when darkness covers him and he slips into a malevolent persona that is rather contrary to the tender and loving person Kamui and Kotori both love. In these moments, he appears uninhibited and cruel, almost sadistic, and he torments Kamui. Then the feeling passes and he claims to have no memory of the previous moment. Still these moments are incredibly ominous.
Finally, Kamui makes his choice and Fuuma takes his place as the other Kamui. He threatens to kill his former friend, tortures him, and ultimately murders the person Kamui loved most in the world. His stated purpose at this point is to protect the Earth from the humans that pollute and destroy it, but later he admits that he really does not care about the fate of the world. His true wish is something only Kamui can grant. Whatever that wish might be, he begins to rampage through Tokyo, blowing up buildings and killing people in order to destroy the Dragons of Heaven and the kekkai that protect the world of mankind.
His personality changes not only in the sense that becomes a mass murdering creeper, but also in general characteristics as well. Fuuma was a serious person for whom duty to his family was an issue of the upmost importance. Fuuma is more casual and easy going and he takes a twisted delight in his work as a Dragon of Earth. He even jokes about tasting the food at the best eateries in town before he and the other Dragons of Earth they are all destroyed. Fuuma seemed like he had to hide a lot of his feelings, most likely in order to support his emotionally, as well as physically, delicate sister. Fuuma is generally more aware of his feelings, though he may not be much more inclined to share them. Fuuma has to be the strong one in his family and protect everyone else while Kamui of the Dragons of Earth protects no one.
Kamui of the Dragons of Earth does, however, possess an uncanny ability to understand other people, often more than they understand themselves. He knows a person's deepest desires--the wish that ultimately motivations him or her--and gives them that wish. It is unclear if he is compelled to grant wishes so, or if he simply chooses to do so. In either case, he finds it disappointing that more people do not want to stay alive more than anything else.
Kamui's feelings for Fuuma are naturally very confused. Unlike the other Dragons, he refuses to acknowledge him as "Kamui of the Dragons of Earth" and instead sees him as "Fuuma" under what might be called possession. Fuuma was his most trusted friend and a person who he relied on when he refused the aid of anybody else. When he turned on him and threatened to kill him, Kamui was absolutely broken up inside. In many ways, Kamui may never completely recover from that incident. Now, Kamui claims that he wants to return Fuuma to his original state, no matter what he has to suffer to do so. It seems that Kamui would even be willing to take the blame for Kotori's murder if it would spare Fuuma from having to suffer.
On the other hand, Kamui sees Fuuma hurting and murdering people and feels guilty since he has not been able to stop him. Kamui is the only person who can stop Kamui of the Dragons of Earth, but how can he do that when the person he loves most in the world now is the person he must fight? He decides that he must be willing to hurt Fuuma in order to stop him, but he does not want to kill him, even if failing to do so will make others unhappy. Kamui loves his friend, and hates what he has become, but he really does not understand him very well. Of course Kamui does not understand other people in general, but with Fuuma/Kamui of the Dragons of Earth, the consequences for failing to understand him might be dire.
In the anime, Kamui was killed by Fuuma in the final battle and had his wish granted. Thus, Kamui was able to create a barrier field in form of a perfect sphere.
Fuuma loved Kamui as well, to the extent that they even seemed like family when they were children. Kamui was a kind hearted, sincere, somewhat naive boy who meant the world to him and it was a horrible fate that marked him as Kamui's twin star. He seemed to genuinely want to protect Kamui and help him with his problems, up until he became Kamui of the Dragons of Earth. At that point, he became Kamui's main problem.
Kamui of the Dragons of Earth understands Kamui, perhaps more than anyone else. He tortures Kamui and taunts him, but ultimately he knows what lurks in the depths of his heart. He even offers to eat Kamui's heart, as if doing so would be a gift to Kamui or a way of granting his wish. Kamui of the Dragons of Earth says that he knows Kamui's true wish and if Kamui does not discover it, he can never win. He also says that Kamui is the only person who can grant him his wish, though Kamui does not know what that is either. In any case, the fate of the world hinges on the relationship between these two teenagers, their individual wishes, and Kamui's ability to understand himself and Fuuma.