Sanan Keisuke, 30. Vice Commander of the Shinsengumi. Also, a scientist of sorts. He is described as being aloof and intimitading. He's smart but doesn't brag about it. He's also very kind and can be mischevious when he wants to be. He's often listened to or asked about his opinion about something since he's educated or even the most educated in the group, and is the one to come up with stratagies and tactics. Hijikata, Okita and Heisuke look up to him as an older brother. Though he does end up not agreeing with Hijikata a lot of the time, but still respects him. 

Although he can come off as polite, and friendly, it's best to listen to his words carefully because they can have a lot of bite behind them and will not be kind. He doesn't like people that pity or look down at him. He's very cynical, cherishes his friends, and will do absolutely anything to help the Shinsengumi. Will also try to get a rise out of people subtly. And will usually try to find a way out of situations that don't require killing innocent people at all costs, unless that is the literal only option left.

A voice example for him is Jade Curtiss from Tales of the Abyss, but less of a smart ass.