Zoro is the first to join the Straw Hat Crew, and a swordsman in whom Luffy places immense trust. Aside from Luffy, he's the only one of them with a bounty over a hundred million.
When Zoro was young, he was friends with a girl named Kuina. Together they worked hard at sword training, but then she died in an accident. To fulfill his promise with her, he strives to become the world's ultimate swordsman. That conviction is strong and intense, so he's always training, he likes adversity, and when fighting starts, he risks his life in the pursuit of more powerful opponents.
Zoro's bossy and dutiful, and will willingly risk his life for his fellow crew. He's like the backbone of the crew, as he'll secretly support Captain Luffy, but then cop a harsh attitude when necessary.