This roleplay is meant to follow the the squad 15 at this point we've settled on that he'd need to be deranked to corporal, investigator, or assassin. More over while you say he quickly rose through the ranks of the Shinsengumi it doesn't explain how so especially since its relatively new. The current captains are either decorated Samurai, or highly skilled ronin. More details would probably be for the best.[@Marcus XVI]