According to Watsuki and the historical novels I've been through, he's almost the same as Sagara Sanosuke. Liked to brawl, short-tempered and simply loved a fight. His favourite weapon was the spear. According to Watsuki, before he joined Kondou, Harada was the servant of some bushis, and this lowly status caused him to have a weak side at times. However, he really took care of those under him. (Really manly...) He was also a man who didn't like to fuss, and did things the way he saw it. Kinda like those "tai-kos" (like Cheng E-Kin) in those gangster movies.
One interesting point here: the members of the Shinsengumi frequented the brothels (hey they *were* humans with biological needs too! Just that Serizawa was carrying it way too far) but they seldom attached themselves to the girls, to take them as their legal wives. Harada married his girl, however - something surprising considering his personality, for in those days, you had the choice of *not* marrying a girl even if you'd slept with her.
After he broke upwith the others following the Shinsengumi defeat in the Bushin War, he joined the Shogitai, the same troop in which Yahiko's late father also took part. Harada died in the Ueno War. According to other sources, however, he survived the war and stowed-away to Manchuria, China, where he became a bandit.