Himura Kenshin (family name first). Kenshin was one of the most feared and skilled assassins during the Bakumatsu (his name then was Hitokiri Battousai - trans. Battousai the man-slayer), a bloody revolution that took place in Japan 10 years previous to the start of the series and marked the fall of the Tokugawa.
Kenshin has a dark and mysterious past and bears a cross-shaped scar on his left cheek. He also carries a sakaba sword (reverse blade sword) and refuses to take human life.
As the story begins Kenshin is wandering through the streets of Tokyo and is attacked by a girl with a wooden sword proclaiming that she will vanquish the evil Hitokiri Battousai. As he jumps away he falls through a roof. While laying in the street he shows the girl his reverse bladed sword... no killer could carry that sword!
The story progresses and Kenshin ends up at the Kamiya Dojo, which is run by that crazy sword wielding girl. A crazy cast of characters, both good and bad then begin to show up in each episode and the fun begins.
The regulars at the Dojo are Kenshin, Kaoru, an ex-gangster named Sanosuke, a former pick pocket turned Kamiya Kasshin Ryu prize student named Yahiko, and a "fox lady" doctor named Megumi.
The first season introduces many characters and enemies but the most memorable would be the Oniwa Banshu. They were an elite Ninja group which had taken on the job of protecting an underground opium/military arms dealer. Through this fight Kenshin gains one of his strongest enemies, and strongest allies.
The second season was insane. More of Kenshin's past is revealed mostly through one of the his former mortal enemies, Hajime Saitou. Saitou was part of the Kyoto "peace keeping" group named the Shinsen Gumi. Saitou just seems to push all the right buttons to bring Kenshin back to his bloody past.
Ghosts from the past also rise up and Kenshin travels to Kyoto to face his toughest, bloodiest, and hardest battle to date. He must fight his hitokiri successor, Shishio Makoto, and the Juppon Gatana. The journey to Kyoto is long and the whole crew from Tokyo take their own route to try and find Kenshin. Definitely my favorite part of the series!
The third season on the other hand is sloppy and caused the stop of the series. While Watsuki-sensei (the creator or Kenshin) was hard at work on the continuing story for the Manga the tv studios grew impatient and simply made up a story line for the Kenshin series to follow.