Himura Kenshin was a former assassin of the Ishin Shishi who fought at the beginning of the Meiji Restoration. He went by the name "Hitokiri Battōsai". The title implies he has mastered the technique of Battōjutsu and can draw his sword with lightning speed. Hitokiri was the title given to samurai assassins. After the Revolution, he picked up a reverse-blade sword, sakabato, given to him by a master swordsmith and assumed the life of a "rurouni". At the age of 28, ten years after the Revolution, he arrives in Tokyo where he meets Kamiya Kaoru, the feisty young instructor of the Kamiya-Kasshin-ryu dojo. After saving her from some rather nasty men, invites Kenshin to stay at the dojo, telling him that she doesn't care about his past. [Taken from: http://en.wikipedia.org/wiki/Himura_Kenshin]
Kenshin arrived in the City completely randomly, as if he turned a corner on the way to the marketplace to buy tofu in Tokyo and was suddenly in the middle of the City square. Although he recognized that he was no longer in Tokyo, or even Japan, his first thought was not of his safety, but, "Kaoru-dono is going to be mad de gozaru..."
Kenshin has now been in the City for well over two years with three separate canon updates. Currently he is mid-jinchuu arc (before finding his 'answer'), however various events in the City, as well as Misao's canon point have allowed him to know how everything turns out.
There have also been a number of other RK crew members come and go from the game, including 4 Kaoru's--the first of which he had married in-game. It has lead for some... interesting play for the following three.
He was also bequeathed a small tea shop by our previous Ukitake Jyuushirou upon his death or disappearance from the City.