Sagara Sanosuke: The Fighting Idiot
Sanosuke makes his first appearance at the end of the first volume of the manga as a street-brawler. Although he appears to have a good heart, his love for fighting and a terrible incident in his past have warped his personality. Although at first he and Kenshin get along just fine, he eventually gets hired to fight with Kenshin. To add to the trouble, Sanosuke has issues with the Ishin Shishi and he sees Kenshin as the greatest of all the patriots and therefore a wonderful target upon which he can vent his fury. However, at the climax of their battle, Sano makes an important realization: that Kenshin is nothing like the greedy, self-serving fake patriots he hates, who lied about a new age, at the expense of others, simply to gain power. He sees in Kenshin a reflection of the man he had admired as a child: the captain of the Sekihoutai, Sagara, to whom Sano looked up and whose last name he eventually took.
Once they are past that, Sano quickly becomes a friend, and Kenshin could not have found a better man to have his back. Sano is, undisputedly, Kenshin's right-hand man. He's the one Kenshin counts on to protect the others when he cannot, and the one who, without being asked, would risk his life to support his friends. Although he and Kenshin don't always see eye to eye and he does go his own way and turns his back on Kenshin and the others twice, he always returns, more determined than ever to help his friends.
Being young (he's only 19), Sano's as hot-headed as Kaoru and Yahiko, though he can keep his cool for a bit longer than his younger companions. He still explodes whenever his friends are in danger or if he is insulted. Sano has a number of feuds going on with other major characters. He never really likes Saitou, and he seems to absolutely despise Chou (a murderous sword fanatic who was part of Shishio's coup plot). He also tends to hate pretty much every Ishin Shishi or imperialist he comes across, not counting Kenshin, of course. Sano also has some nice tension going on with Megumi, although nothing is said outright (the flirting is much clearer in the anime) and although he detests her at first (out of principle), they do get along better later. Although their friendship is marred by Sano's stubborn disregard for all the hard work Megumi puts into fixing him up after fights by going out and fighting again.
Sano's fighting style is quite different from the others. Although he starts out with a badass sword, he fights barehanded for most of the series. He doesn't seem to have had any formal training in kenjutsu -- or any other jutsu, for that matter (insert your own Naruto joke here). His fighting style relies more on brute strength and endurance rather than skill. The one technique he does learn -- the Futae no Kiwami which can pulverize pretty much anything -- he just sort of picks up, and he never really masters it. I wonder, though. Makes me think if Anji met Cologne somewhere in the mountains... (Bad Ranma joke...oh, forget it.)
Although he calls himself the fighting idiot (and Saitou treats him just like one all the time), he's actually pretty sharp - just terribly inexperienced. Also, even though he thinks Kaoru is naive, I think that he has a different brand of naivety himself. Sano doesn't have the kind of cunning, or the insight that recognizes cunning, that Saitou and Kenshin have -- which is probably why he says and does some really "stupid" things in the Kyoto arc. He simply doesn't know how to fight "dirty," because unlike Saitou and Kenshin, he has never had to truly fight for his life.
All in all, Sano's not the kind of guy who sits and meditates and thinks things through -- he's more the "bash faces in and get it over with" type. He makes up for his lack of technical skill and experience with his astonishing endurance, resilience, and natural pig-headedness.