A demon working under Kazama's orders. He claims in episode 7 that he and Kazama are repaying their debts to Satsuma clan.  Source: episode 7 of Hakuouki TV series

The protagonist, Yukimura Chizuru, is the daughter of a doctor who works in Edo. The father leaves Edo to work as a volunteer doctor and moves to Kyoto without his daughter. As time passes by, Chizuru starts worrying about losing contact with her father, so she decides to go to Kyoto in search of him. On the way, Chizuru is attacked by few criminals and witnesses a fight between an oni and the Shinsengumi. Taking her into custody and saving her, the Shinsengumi debate on what to do with Chizuru when they discover that she is the daughter of the doctor they are also looking for. So they decide to become Chizuru's protectors and help her look for her father (the doctor). Lots of events happen while she stays with the Shinsengumi, as they discover mysterious secrets and also fight against the Bakumatsu group. The story is overall romantic with a historical and political background. 
(Source: Hakuouki official website)

The continuation of Hakuoki. 
The Edo Era has ended after 260 years of history and the Meiji Era begins. However, bitter strife still continues between the soldiers of the new government and former Tokugawa retainers. This is the stage in which the surviving members of the Shinsengumi find themselves upon their return to Edo following their defeat at Tobafushimi. 
(Sources: ANN, AniDB)

This OVA takes place in between episodes 8 and 9 of the first season of Hakuouki. 
Yukimura Chizuru goes on a reconnaissance mission in Shimabara as a geiko for the Shinsengumi. Each episode unfolds the events of the days leading up to, during, and after the operation. It intermittently reveals each captain's involvement as well as that of a few demons in the affair. Each individual episode primarily focuses from the third person perspectives of: Okita Souji, Saito Hajime, Harada Sanosuke, Toudou Heisuke, Hijikata Toshizo, and Kazama Chikage.

Based originally on the famous otome game, this series is Ninomiya Sachi's version of Yukimura Chizuru's story as she arrives in Kyo from Edo to search for her missing father and accidentally meets the members of the Shinsengumi, who harbor an awful secret.  Unique from other versions, this series unfolds a bit out of order, starting already at the Ikedaya-jiken and then progressing with flashbacks.

Based originally on the famous otome game, this series is Hiraku Naki's version of Yukimura Chizuru's story as she arrives in Kyo from Edo to search for her missing father and eventually becomes involved with the Shinsengumi.  This version begins the night that she meets the members for the first time.  

A novel adaptation of the game "Hakuoki", following Hajime Saitou's route.

Members of shinsengumi are students or teachers.

Kyuujuu Amagiri