Member of the Bakumatsu, enemies of the Shinsengumi. He is the strongest Oni of the West and is after Chizuru, so that they can mate and create stronger Oni. His reasons for this are unknown, but his attempts to take Chizuru have so far been thwarted by the interference of the Shinsengumi. Despite being arrogant and cynical, he is not entirely cold-blooded, as he later saves Chizuru, although this might have been for his own purposes. Kazama is usually followed by his two demon companions Amagiri and Shiranui, and it is implied that he might be of royal blood, like Sen-hime. In his Oni form, he grows white horns and his hair fades to match. His eyes also become yellow.
Video Game Spoilers
In the otome game that the anime and manga were based on, Kazama is one of the six characters that Chizuru could end up with based on the player's decisions throughout the game.
It's stated in the game that Kazama is of pure demon blood and he's the head of the Kazama family that reigns over the west. The reason why he's after Chizuru is because she too is of pure blood, and if they mated it will result in an even more powerful demon child.
His route in the game also reveals that he's a man with a high sense of honor, and the reason why he sided with Satsuma was to return the debt for protecting his clan during the demon extermination issued by the shogun. It's also stated that the reason why he attempts to kidnap Chizuru only when she's under the protection of the Shinsengumi is because he wants to shame the Shinsengumi after defeating them in a clean fight, of which Sen comments that he's acting like a classic villain.
However, his actual reason is because he just wants to have fun with the Shinsengumi and uses Chizuru as an excuse to go and mess with them. He explains to Chizuru that he wouldn't have stuck around as long if he had thought she was just like a regular female demon and that during their time together, he found her interesting and will never be bored with her around.
(Source: Main Route - Hakuouki: Demon of the Fleeting Blossom, Fandisc Zuisouroku - Hakuouki: Stories of the Shinsengumi)