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Wounds suffered before becoming a vampire will not heal.
--Kamui (Tsubasa Tokyo Revelations Episode 3: The Princess' Dream)
Kamui in Tsubasa Chronicle
Subaru (twin brother)
Tsubasa Chronicle Chapitre 108: The Country on Shifting Sands
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Tsubasa Chronicle Chapitre 233: Epilogue- A world to return to
Tsubasa Shunraiki Episode 2: The Latter Part
Kamui is one of the Vampire twins that Seishirou was after. Kamui makes his first appearance in Tsubasa: Reservoir Chronicles chapter 104, manga volume 14, at which point he shoots a stranger (Syaoran) in the leg for the crime of entering a building. The very first line he says is, "The fact that you've stepped foot inside here means that you want to die." (vol 14)
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.
The one thing they share in common with the common vampire is that they feed on blood. Also the clothes Kamui and Subaru wear (not including Kamui's Acid Tokyo outfit) appear to fit a certain Byronic sensibility. That seems to be the extent of their relationship with the vampire canon. In a lot of other ways, they differ considerably from other vampire stories.
Perhaps the main point of difference is the fact that the vampires here are not immortal. Certainly other vampire stories suggest methods for killing vampires which means that those vampires are not entirely immortal, but in TRC, could potentially die of old age. Instead of immortality, the vampires have considerable regeneration abilities so they heal very quickly and can survive damage that would kill other beings. Subaru and Kamui are described as "pure breed" vampires, a title which is not explained but probably means they were born from a vampire, perhaps both a vampire mother and father. As such, they are particularly strong and able to heal. Those who become vampires by drinking vampire blood simply age slower and heal faster--they are not as nearly indestructible as pure breed vampires. As Kamui explains, "Wounds suffered before becoming a vampire will not heal." (vol 17) The text rejects the idea that vampires of either type have an aversion to holy water or sun light.
Of course vampires have other abilities besides healing and regeneration. As Kamui demonstrates in his fight with Kurogane, vampires can move very quickly, making them hard to hit. They are also capable of producing in an instant massively long and horribly sharp claws that they can control with razor precision. These long, sharp nails are their main weapons in battle. Although it is unclear if it gives them an advantage or not, when a vampire accesses his or her powers, his eyes become more animal like in appearance with a slanted pupil. Perhaps this allows them to see better or with more precision, but this is never explained. While common vampires are often fast and may have animal like eyes at times, they usually do not have retractable fingernails they use to fight.
The most interesting difference between regular vampires and TRC vampires are their eating habits. Regular vampires consume blood, and while usually any blood will do, some require the blood of virgins. In TRC, vampires consume the blood of what is called "E" or "prey"--artificial beings created for the purpose of feeding blood drinking creatures. It is because Syaoran's clone is an artificial being that Kamui thought that his blood might allow Subaru to wake up from his enchanted sleep. Apparently regular people can become "prey" as well by mixing their blood with the vampire blood used to turn another person. If this is done, that person will only be able to drink the blood of that specific person, meaning that their ability to live is tied to the "prey". Another way in which the vampires are different is that they do not appear to have fangs, so they do not create wounds by biting their prey. The text generally avoids the typical image of the vampire feeding on the person's neck and instead has the vampire make an incision on the person's wrist, which may be a little less risky than slicing the jugular and is probably a little less sexually suggestive. That does not mean that a vampire could not feed on a person's neck, but it may have been an aesthetic choice on CLAMP's part.
The Government Building group consists of six individuals, plus Kamui, who were responsible for guarding the water reservoir under their building. They are responsible for the lives of all the people who live in the Government Building--one of the only structures that withstood the acid rain. The members are the same as the Dragons of Earth from X, except Seishirou has been replaced with a young version of Kazuki. They are opposed by the Tower Group, a rival gang that resides in the Tokyo Tower supported by another water source. They are made up of the Dragons of Heaven, except Subaru is replaced by Daisuke.
Despite living with this group of people for two years in relative isolation and working together through extreme circumstances, Kamui develops only the vaguest relationship with this group. While he may lead them, he does not particularly care about them or their plight. His concern is only with the well being of his sleeping companion. When Subaru finally comes out of the cocoon, Kamui insists on leaving right away despite the dire circumstances the Government Building people are in. It is only his brother's sense of compassion and responsibility that keeps him from leaving them without the water they so desperately fought to protect.
The Government Building Group is not quite so detached in their estimation of Kamui and they look up to him as a leader. The others sense his sadness, but none are able to get close enough to him to discover its source. They realize how important his aid is in their struggle to protect the water, so they listen to him and try to aid him whenever possible. Nobody reproaches Kamui for wanting to leave, but they also realize how desperate their situation will become when they lose him. Fortunately Subaru's deal with Yuuko and Fuuma's intervention assure the survival of the Group and the people who rely on them.
Kamui's interest in the Tsubasa group, the travelers looking for Sakura's feathers, appears to be directly proportional to the degree of their perceived threat to the reservoir. When they appear in the Government Building and seem to be water thieves, Kamui tries to hit them with his arrows. Once they demonstrate some fighting skill, particularly Kurogane (though he does take interest in Syaoran's fighting technique), his interest increases. Once a bigger threat shows up (the Tower Group) and confirms that Tsubasa Group is not with them, he loses interest in them. It is not until Sakura's sleeping spirit resonates with her feather and disturbs the cocoon in which Subaru was sleeping that Kamui takes an interest in the again.
For the most part, his attention is focused on Syaoran (or what turns out to be his clone since) he clearly knows Seishirou (since Seishirou trained him in his unique fighting style), which makes him potentially more of a threat. He also knows that Syaoran the clone is a created being, so Kamui decides to feed his blood to Subaru in the hopes that it would revive him. This does not go as planned, but in the end, the clone ends up breaking open the cocoon and setting Subaru free, so, to that extent, the Tsubasa group was helpful to Kamui.
When the time came for the twins to help the Tsubasa group, Kamui volunteers, not because he feels particularly moved by their plight, but to prevent his brother from getting involved again. He gives Fai his blood to save his life and explains some the finer points of the vampire powers and limitation. He seems surprised when he learns that Kurogane agreed to be Fai's blood supplier before knowing exactly what that meant. Perhaps this illustrates that does not grasp the bounds of friendship.
In any case, his relationship with the Tsubasa group is limited by not only their brief time spent in Acid Tokyo, but also by his own taciturn disposition. He has a powerful impact on the group by turning Fai into a vampire and saving his life, and they in turn, set Subaru free which allows the twins to move on.
Fuuma is Seishirou's "brother" (though they are not blood related) and another person who travels through the dimensions hunting for various things--in his case, generally artifacts of value. He trades with Yuuko, the Space Time Witch, for the ability to travel to different worlds on what he refers to as a "pay as you go" plan. His exact purpose for being in Acid Tokyo is never explicitly stated, but he might have been planted there for the purpose of helping Sakura. Unlike Kamui, Fuuma actually seems to care about the people around him, at least enough to broker a bargain between the two sides as well as provide Sakura with the tools she would need to traverse Acid Tokyo. Of course, this is not to say that he is an angel. Yuuko points out that both he and his brother have a penchant for causing mischief.
Fuuma is probably the only person in Acid Tokyo who is a match for Kamui, a fact that seems to vex Kamui and amuse Fuuma. To Kamui, Fuuma is an enemy who must be defeated in order to protect the reservoir and Subaru's resting place. Once he learns of his kinship with Seishirou, Kamui seems to hate him even more. Fuuma, on the other hand, finds Kamui amusing and approaches their fights with a distinct sense of playfulness. When he manages to get the better of Kamui by provoking him, Fuuma holds him in a rather suggestive manner and says, "You're just as big brother described you. You are strong, aren't you, Kamui?" (vol 17) Fuuma seems to have a pretty good understanding of Kamui's personality, just like his counterpart in X. Some of this insight may be due to learning about him before hand from his brother, or it could be that reading people is one of his talents. When Kamui informs him that he will be leaving, Fuuma seems surprised, perhaps even a little disappointed. Although Kamui does not seem to like him very much, his time in Acid Tokyo would have been a lot more boring without Fuuma to challenge him.
On any other topic, Kamui tends to remain cold and distant (except for when dealing with Subaru), but just the thought of Seishirou is enough to send Kamui into a rage. He refers to him in a sort of "that guy" fashion, his tone dripping with wrath, contempt, and perhaps a little fear.
According to CLAMP, in Tsubasa: Reservoir Chronicle, people are the same no matter which dimension they are in. Certainly Vampire Kamui shares some characteristics with his X universe counterpart, but there are actually several differences between them, if one looks beyond the Kamui of the first few volumes. Of course, given that we know so little about Vampire Kamui's past, it is unclear whether the difference is in their intrinsic dispositions or a matter of circumstances. It is quite likely that the two have lived considerably different lives, so it makes sense that their personalities would diverge in some respects.
Chapter 109 of Volume 15, Child of God was initially made for him and featured to his own formal introduction. In the said chapter most of his hidden personality is slowly revealed. His name also translation is Child of God.