Saitou Hajime: The Wolf of Mibu
Saitou Hajime is just one of the many blasts from Kenshin's past that turn up, but he is the first who sticks around and plays a very important role to the very end of the series. He, too, is a veteran of the Bakumatsu, although he fought on the Shinsengumi side. Saitou Hajime is also the only real "historical" character, based on the real Saitou Hajime of the real Shinsengumi.
Although he was a supporter of the shogunate, he works as a policeman in the Meiji period, safeguarding the very government he fought against. This may seem very odd but I can understand the logic behind Saitou's transformation into Fujita Gorou. He decided to keep an eye on the government and the people that fate and time chose to put into power, all for the sake of the country. Saitou hates corruption and evil in all its forms, which is why he was able to "switch sides" so easily. He is, I think, very much like Kenshin in this sense. Their swords don't really serve "sides" or factions but their own principles of what is right and what is wrong.
However, one mustn't make the mistake that he's just as "enlightened" as Kenshin. Saitou's philosophy is brutally straightforward and unforgiving. His principle, "Aku zoku zan," means to kill evil instantly - the opposite of Kenshin's (everybody deserves a second chance).
Saitou's fighting style is not battou-jutsu. He starts off with his sword already unsheathed and directs it in an angled upward thrust. He pierces more than he hacks and slashes, but causes enormous damage, whatever he does. Unlike Kenshin, he is not averse to killing, although his attacks can transform from a thrust into a slash immediately upon missing. Most unsettling is how he sometimes seems to take it in stride. Though he is far from being a wanton murderer, killing only those who he deems to have earned his own personal brand of justice, I don't think he's the type who would lose much sleep over it. Still, he must have some softness inside him, as evidenced by the boy, orphaned by Shishio's minions, that he (presumably) took in.
I like Saitou a lot. HEAPS. He's a strange guy whose morals are a little skewed but whose motives are always clearly good - in a general sort of way. I mean, Saitou's the type who would always look at the "greater good" or what is lawful. Even though he considers himself separate from the Kenshin-gumi (i.e., he's not "on their side," strictly speaking), he is mostly treated as an ally. Of course, in Rurouni Kenshin, friends and enemies are equally given the shinai-in-your-face treatment, so it's really hard to say.
Saitou disdains Kenshin's efforts to suppress his hitokiri side but is forced, time and time again, to acknowledge Kenshin's skills as a swordsman. In the end, however, Saitou realizes that Kenshin's true nature is that of a gentle vagabond. Although he acts as though Kenshin has severely disappointed him, I can't help but think that what he really feels toward his former rival is respect. Grudging, but respect all the same.