A thing that have to be recognized about Zoro, is that Oda was able to made with him a very unlikely thing in manga. Some people compare Zoro with Vegeta or Sasuke, in the idea of them being the "rival" of the protagonist, but thats totally nonsense, Kid, Law and expecially Teach are those rivals for Luffy, the place Zoro take in the story is much more akin to Shikamaru or Killua, the buddy, the friend that will stay at the side of the protagonist no matter the problem, but while normally this buddy start to get behind the protagonist and lose space in the story, like how happend with Kuririn, Ishida and Renji. Oda was able to made a character that normally take a side position to become a central one, and one of the most loved, and make beliaveble that he can stay at Luffy side.