Hakuouki - Season 1

Yukimura Chizuru has come from Edo to look for her father in Kyoto. She ends up being chased by a couple of rounin, who are cut down by some evil looking Shinsengumi, who are then killed by some regular Shinsengumi. Hijikata Toshiro is there, and decides to take Yukimura with them instead of killing her on the spot. She ends up living with them, disguised as a boy, and helping them in anything she can.

Gotta throw a wrench in there, though. Turns out that Yakimua's actually an oni. Yukimura's father, who is a doctor, helped create a medicine which turns humans into a Rasetsu, or pseudo-oni. The Shinsengumi are using the Rasetsu as their trump card. All this goes on during the actual historical events, modifying things here and there. Very cool.

The current season is over, but a second season will be airing in the fall. Yay!