To start, Kenshin's real name was actually Shinta, given to him by his parents that died of cholera shortly after Shinta was born and he was picked up by a slave trading family. The group of slave traders were attacked by bandits on the road. Three sisters from the slave trading family protect Shinta (Kenshin) with their bodies as shields as the bandits started killing them off.
The 3 sisters died gruesomely, but Shinta was saved. Hiko, the only swordsman who knew the Hiten Mitsurugi (a form of swordsmanship that is the most powerful in all of Japan), happened to see the bandits attacking the family and decided to save Shinta. After doing so, Hiko left Shinta alone only to return later on seeing that he had made graves for everyone even the bandits. His explanation was that people are people, but when they die they're just corpses.
Hiko realized that Shinta had potential in him as a swordsman and adopted him as his pupil and the next generation of Hiten Mitsurugi. Hiko believed Shinta was too nice of a name for a swordsman and renamed him to Kenshin which means "heart of sword."
Kenshin trained hard wanting to be able to someday protect people with his sword. A couple years pass and Kenshin still wants to help people. He questions his master's reasonings for wanting to stay isolated from everyone else when they both had the power to help people. Hiko explains that Kenshin will become a political pawn if he leaves. A war is raging on in Japan; clans of people that want to abolish the Tokugawa shogunate are popping up. They don't see eye to eye, so Kenshin thanked his master for training him and departs after a fight.
Kenshin joins the Ishinshishi, fighting to bring on a new era in Japan. In order for a new era to begin, it must be first thrown into chaos. Kenshin has the cruelest role of all acting as a guard for the code of disorder. From time to time, Kenshin recieves a slip of paper with a name on it. He must kill that person in the shadows of the night (he always succeeds ^_^) He become famous in his clan and earns the nickname "hitokiri battousai" (shadow killer). No matter how much he killed, the succession of a new era wasn't getting anywhere. As a whole, he was just being a killer over and over again...
He meets a beautiful girl named Tomoe Yukishiro, who was the future wife of one of the Shinshengumi members he had killed earlier. He was also the guy who gave Battousai his first scar on the left cheeck. His love for Tomoe and the will to live made him almost as strong as Battousai, giving him the strength to fight. To shoten the story a bit, Tomoe started living with Kenshin in the Ishinshishi's secret hide out (which was a hotel). Later, a mole in the group lead to the murder of Ishishishi's leaders. The other memebers went into hiding. Battousai was orders to keep it low, with Tomoe as his wife. Then, Kenshin proposed to her and they began to live together in a small village where only qualified Ishinshishi knew. All was going well for a while for the two couple, Battousai was actually smiling and Tomoe finally knowing what happyness is. After certain events Enishi has shown up to check up on his Sister, Tomoe and to hire some assassins to kill Battousai. You see, Tomoe originally was sent out to find out Battousai's weaknesses and to spy on him but she actually fell in love. Enishi, when he found out was enraged and went out to hire 4 assassins to finish Battousai of once and for all. Sadly, when Battousai was battling he had accidentally slashed Tomoe in the back killing her and getting his secong scar forming an X on his left cheek. 10 years pass and the year is now 1878.. Kenshin acquired a sakabatou (reverse blade), a katana with the blunt and sharp edges reversed. He makes a promise not to kill again...and wanders around Japan. He finally stops in Tokyo after he meets a girl named Kaoru. That's where I'll stop! When Kenshin meets Kaoru it's where the story begins in the manga and TV series.