He was originally a bounty hunter who was tied to pole for killing Helmeppo's pet wolf that was attacking a little girl. Luffy intended for Zoro to join him, but he refused as he was a bounty hunter and thus hunted pirates. After learning however, that he was to be executed at the end of the month, Zoro turns on the Marine captain, Axe Hand Morgan and joined Luffy as the first crew member.
He desires to become the world's greatest swordsman and hlds an immense amount of honor. Despite Luffy's goofy antics, Zoro holds his captain with deep respect and willingly trained with his rival Dracule Mihawk for two years to help his captain.
Despite his lean figure, Zoro is immensely strong (which had grown even further after the timeskip), capable of lifting even a building. He is the master of the three sword style technique and thus uses three swords in battle (one is held by his mouth).