Hakuouki: Shinsengumi Kitan.

Ochimizu:

Ochimizu is a special potion, giving the user the ability of extreme regeneration, even healing fatal injuries. However, it has very severe side effects; becoming bloodthirsty and taking on a demonic state, with white hair and red eyes, and feeling weak in the sunlight. Developed by Isami Kondou under the Bakufu's order, it was tested on the Shinsengumi's lower officers. Later on, the main Shinsengumi members take ochimizu to survive dangerous injuries they had sustained from fights with Chikage Kazama and his cronies. As of season one, the only members of the Shinsengumi that didn't take ochimizu yet are Hajime Saito, Shinpachi Nagakura, and Sanosuke Harada.

Rasetsu:

People that take ochimizu are referred to as "rasetsu". Ever since he took ochimizu, Keisuke Yamanami is the current leader of the Shinsengumi's rasetsu unit. The Shinsengumi and the Bakufu are the only people that know about the unit. It can be considered as a secret weapon.

Rasetsu are not immortal; they can still be injured and killed. Silver weapons, such as bullets, can prevent the rasetsu from regenerating. Furthermore, the rasetsu lose their ability to think logically in battle when they become bloodthirsty, causing them to recklessly fight in battle. Recently, Sannan-san has developed a powder that temporarily stops a rasetsu's need for blood, although when he gives it to Heisuke he tells him that eventually he will have to consume blood to survive as a rasetsu.

In the second season Amagiri Kyūju, one of Chikage Kazama's henchmen, reveals that the Rasetsu are shortening their own lives by using their powers, which slowly consumes their life force. Due to this, Toshizo Hijikata puts the Shinsengumi Rasetsu Unit out of commission, much to Keisuke Yamanami's irritation.