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In some ways, that sums up Kamui's character--cold, hostile, and decisive. Yet as his story unfolds, at least in a limited respect, it becomes clear that there is more to Kamui just the surface. He might not be very kind hearted, but he is not a monster either. Instead, he is absolutely devoted to his twin brother, Subaru, and would go to any lengths to keep him protected.
That is not to say, of course, that the initial impression Kamui gives in his introductory chapter is entirely wrong. He is cold towards nearly everyone, even the people he has lived with for two years, never really opening up and forming connections with them. Under most circumstances, even when engaging in battle, he remains distant and aloof, as if any pain suffered by either side has no meaning. He is, however, not without emotions and it seems that his comrades are able to understand him, at least to some extent.
Although he may seem aloof, he is also violent and incredibly quick to engage in a fight and he does so without a sense of mercy. If he decides someone is a threat, he is determined to eliminate him or her as quickly as possible. He does not appear to take particular pleasure in hurting others, but he also does not seem to find it distasteful. He is not without reason, however, and will let people live if he decides they are not a threat. He told his companions to "take care of" the Tsubasa group, and while he may have meant "kill them", he did not seem upset when his group provided them with aid and it seems unlikely that his group would tempt his anger if they thought he really wanted them dead.
If anything can be said about Kamui it is that he is decisive. Unlike his X counterpart, he acts with vicious efficiency and shows no hesitation or confusion. In a way, this makes sense because Kamui of X is both emotionally driven and emotionally unsettled, where as Vampire Kamui is driven by a very specific desire. His decisive tendencies and confidence make him an effective fighter and leader, but he loses some of his humanity in the process. Also, his sense of determination ties in with his unwillingness to lose, which Fuuma manipulates with startling efficiency.
While Kamui certainly possess some of the qualities of effective leader, his leadership of the Government Building group is of a very specific type. He leads because he expects people to follow orders while leaving him alone to do whatever he wants to do. On the one hand, he is confident, determined, and decisive, which might be why the others follow him. Certainly they respect his strength and his ability to help them achieve their ends, but they also seem to admire him as well. On the other hand, his leadership is limited by his lack of concern for those he leads, making him something more like a ruler than a guide. Still, his position of leadership seems to be a mutual understanding, resulting in a symbiotic relationship between Kamui and his comrades.
Kamui lacks a fundamental sense of trust in anyone except his brother. Perhaps that is why he remains so aloof--he trusts no one enough to make a connection with them. It may also relate to why he is so protective of Subaru--since he is the only one he can trust, Kamui would be completely lost without him. Something that the Government Building group says seems to support this idea. According to them, when Kamui first appeared in the reservoir, he was a savage killing machine and it took a long time for him to stop being hostile. Although they were unaware of this fact, he was fighting desperately to protect his brother from what he assumed was a threat. They did observe at the time, however, that he seemed like a lost child. With no one to trust, he had to act entirely on his own and failing meant losing everything he had. Due to his separation from his twin and the growing risk of Seishirou catching up with them, Kamui most likely felt very lonely and anxious.
No one is more important to Kamui than Subaru and it is only towards him that Kamui shows any extreme emotion, with the exception of Seishirou. In Seishirou's case, however, that emotion is burning, unyielding hatred tinged with fear and desperation. Even the mention of Seishirou's name can send Kamui into a volatile and reckless rage. Kamui's life seems to have one primary purpose--to keep him and his brother away from Seishirou and whatever he plans to do to them. Those plans are generally mysterious, but they are serious enough to make Kamui, who is extremely strong and capable of incredibly fast healing, scared enough to pay the high price to travel the dimensions in order to avoid him. To Kamui, keeping Subaru with him is worth whatever the cost. He certainly spent two years guarding his resting place and frantically hoping to revive him in some way. When Subaru offers up his blood to help others, Kamui insists that he will give his blood instead, in order to prevent another situation like the one with Seishirou.
The fact that Kamui is so savagely protective of his brother might be the sign of a disturbed and jealous mind, or it could be that he has very good reason to be so vigilant. It is possible that their life up to this point has not been particularly carefree. Vampire Kamui may not have suffered the exact horrors of his X counterpart, but that is not to say that he has not suffered in his own way. Given the powers of the vampires, it is not difficult to imagine people trying to hunt or attacking them for their blood, especially since they are pure bred vampires. Perhaps Seishirou is not the first to hunt them, but rather the most dangerous and potentially successful. A life spent looking over one's shoulder for potential danger could make someone rather hostile and over protective. Given that little of their background is explored, this is almost purely speculation, but the fact that Subaru is also very protective as well seems to support the idea that they have reason to be this way.
It would certainly be nice to see Kamui's character under less dire circumstances in order to get a fuller picture of his personality. As it is, Kamui clearly cares deeply about his twin and most of his actions revolve around that fact. Perhaps some aspects of his personality are also determined by this fundamental desire to protect his brother, in that the person he has become developed out necessity of fulfilling that goal. At any rate, his absolute devotion to Subaru demonstrates that is not as cold as he first appears.