Saitou Hajime seems to have struck second fame in Japan following his appearance in RK! There are certainly a lot of info about him on the web. Seems to be a mysterious guy even in real life... Also, everything about him that came out in RK (except Shishio and Kenshin, of course) is real history. I got a shock there!
Right. First of all, his origins seems a bit obscure. One theory is that he's the son of a ronin. Another says he hailed from Edo, son of a Bakufu official, and that he'd come to Kyoto after accidentally killing a person in Edo. Yet another claims that he's a son of the 12th Shogun?! and yet another says that he was a spy of the Aizu han placed in the early Roshitai. Talk about a mystery guy. His name, Hajime (meaning "one" or "start" as in another kanji") was because that he was born on the 1st day of the 1st month, though there are other reports as to his birthday.
He entered the Shinsengumi "sponsored" by the Aizu han, under the name of Yamaguchi Jirou, when he served as one of Hijikata's assistants. His most powerful sword skill is the "Left-hand single thrust" (but *not* Gatotsu, that's Watsuki-san's creation), this skill is even more destructive than Okita's 3-piece thrust! In fact, Saitou, together with Okita and Nagakura, were hailed as the 3 strongest swordsman of the Shinsengumi. Saitou assasinated a lot of corrupted Shinsengumi members. Eg, Itou Kashitarou and co., Takeda Kanryuusai and others. And yeah, Aku.Zoku.San was no invention of Watsuki - it was a motto of the whole Shinsengumi!
He was also a great drinker! The creator of one Shinsengumi website stated that "he's a different person when drunk", (I'm not sure whether this refers to "I desire to kill once I'm drunk"). In fact, he finally passed away because of a stomach ulcer, caused by excessive drinking. Unfortunately, I still can't verify that he smokes all the time, or that his favorite food is soba... help, anyone? Saitou's wife, Takagi Tokio, was the daughter of an important Aizu official - in fact,the middle-man was none other than the Daimyo of Aizu himself! They (Saitou and Takagi Tokio) were married in the 6th year of Meiji (1873), and their first son, Tsutomu was born in the 9th year (1876).After the Ishin government was formed, he changed his name and worked as a kendo instructor in a university. In 1877, the 10th year of Meiji, he joined thepolice troop with permission of carrying a sword to fight in the Seinan War, the one where Saigo Takamori rebelled against the Meiji government in Satsuma. Saitou did this under the name of Fujita Gorou. After which he became a spy directly responsible to the Police Commissioner, Kawaji Toshiyoshi. (I was really shocked at this one... Watsuki can really interweave fact and fiction!)
Saitou worked as a guard of a museum in Tokyo when he was quite a senior citizen.After his retirement, he seemed to spend a lot of time thinking. Pity I can't understand his grandchild's remarks in the Japanese homepage. Anyway, Saitou passed away on 27/9/1915, 4th year of Taisho, at the ripe old age of 72.