Oh! Rurouni Kenshin is an anime and manga about an assassin working for the other side, so one of the Shinsengumi's worst enemies. It takes place ten years after the end of the war when he's trying to come to terms with everything he did and the new world. It co-stars middle-aged police officer Saito as an occasional antagonist and occasional reluctant ally.

Unfortunately I can't really recommend it because the creator is a pedophile (was convicted of possessing child pornography) and you can sort of tell in the work. The women characters are much younger than the men characters and while the men are all survivors of the war and dealing with its effects the women were too young to be in the war and their arcs mostly revolve around the men. So it is a very flawed piece of work. But there are some very good things in it, including its take on Saito.