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Kazuki Mutou
Voice actor: Jun Fukuyama
Height: 170 cm
Weight: 59 kg
Birthday: December 1
Blood type: 0
Age: 16
Favourite colour: orange
Likes: curry rice, aojiro, reading manga
Dislikes: shiitake mushrooms, Mathematics
Special skill: several, actually, I'm a pro at [insert category here]!!
Affiliation: Ginsei Private School, class 2-B, boarding student
(Source: Busou Renkin #1)
Busou Renkin's main character is sixteen years old, friendly, helpful and so energetic that it borders on silliness.
Despite his fear and Tokiko's insistence that he does not have to, he joins the girl in her battle against the homunculi. Kazuki's motivation is simple: he just can't look away and pretend that everything is as before.
'Simple' is indeed the right word for Kazuki: He is rather straight-forward and by no means the greatest of thinkers. And he isn't going to let even the bleakest situation mar his optimism. Contrary to Tokiko, Kazuki can easily feel compassion for his enemies.
His driving force is the wish to protect others. This is what enables him to overcome his fear of getting injured or killed, and motivates him to fight alongside Tokiko.
Concerning his creation, Watsuki mentions that his personality was partly based on that of Rurouni Kenshin's Misao Makimachi. The similarity is definitely there.
I keep reading on forums and elsewhere that Kazuki makes people think of Yahiko, but in all honesty, I can see no parallels. Their hair, if you look closely, is not the same, even though it is true that both have what one would call 'messy black hair'--but that's hardly rare among shounen manga characters.
The concept that Watsuki used for Kazuki is: normal, outgoing student with a great sense of humour and a lot of energy.