Next up, Zoro!
I really find Zoro's character growth to be one of the most interesting in the series. He starts off to all appearances as a typical lone wolf, with his own ambition the most important thing in his life. But I don't think that view is entirely correct; he wants to be the strongest swordsman, true, but I think to him "swordsman" means a lot more than "some guy who fights using a sword". He follows a warrior code; one that he would die before breaking. Vivi names him well when she calls him "Mr. Bushido". Like Luffy, fulfilling his dream means nothing if he can't do it on his own terms.
I strongly believe that similarity is at least part of why Zoro changes his mind about joining. Sure, Luffy saving his life was part of it, no doubt, but I think what really intrigued Zoro is Coby's description of why Luffy is trying to save him. I think that Zoro saw his own way of thinking reflected in that story; Luffy was willing to let things stand until Helmeppo went back on his word, only then did he interfere. I think that struck a chord in Zoro, a realization that Luffy might be cast from the same mould as himself; a belief that is further strengthened by Luffy's instant acceptance of his condition for joining. And then Luffy takes things a step further by showing Zoro that he fully trusts him to have his back.
Right from the start, there's a bond of understanding and trust between the two, though their personalities are almost polar opposites, they understand each other much more than most of the other members of the crew. They are the only two real warriors, both following the unspoken Code. But even more than this understanding, what really helps Zoro to grow past his lone wolf beginnings is Luffy's absolute trust in him. Luffy basically treats Zoro as he does himself, trusting that Zoro is capable of handling almost anything. In fact, prior to Shabondy, the only time we see Luffy worried about Zoro in a fight is during the duel with Hawk-Eyes, and then, even though he was worried, he understood the importance of following the code and preventing himself and anyone else from interfering.
And I think that level of trust and acceptance really inspire Zoro. Even though he insists at the beginning "I'll do things my way", by the time of the duel with Hawk-Eyes, he's already started to intertwine his ambitions with Luffy's. We see that Luffy's approval is something important to him. I think it's combination of two factors, firstly, that Zoro admires Luffy for what he is; his ambition, the way he treats his crew, his lack of hesitation, etc. But even more importantly, I think Zoro sees what Marco sees at Marineford; that "great potential" waiting to awaken. And that dual way of seeing him leads to a rather interesting dynamic between the two; it's almost like Zoro is the older retainer for a young lord. Zoro is Luffy's right hand, willing to do anything for him, and at the same time, there to give advice and mentor him to the next level.
Because, as hilarious as the recurring gag of "why isn't he (Zoro) the captain?" is, there's an element of truth to it - Zoro is the better natural leader. But he tries his best to help Luffy grow into the role as well. For a long time, that seems to be the focus of his attention; Zoro pays attention to the rest of the crew only by virtue of their connection to the captain. He's the last to accept a new member, and the first to get angry over perceived betrayal. He may trust Luffy, but he doesn't necessarily trust his judgement of other people. However, I think by the end of Water 7, that changes. Zoro sees real change in Luffy's leadership capability and sees Luffy's judgement vindicated. From that point on, I think Zoro sees Luffy-as-he-currently-is as a person worth following, not just as a potential Luffy-as-he-will-be. While he still defines himself as Luffy's man, he begins to relax and settle into his role in the crew, finally beginning to accept them on their own merits. (and beginning his next project, but that's the topic of another article^^)
In a lot of ways, I think Zoro's initial character journey is over; from the man who told Luffy, "If you get in the way of my ambition, I'll kill you!" to that incredible scene of "Instead of Luffy's head, take mine instead... for Luffy is the man who will be the Pirate King!". He still has a long ways to go though; I expect we'll see him continue to grow more attached to the crew as a whole, and I think we'll also see his ambition continue to change. I think he begun to truly see what Luffy meant that day back when they met, and that being the strongest for the sake of being the strongest is meaningless, regardless of the secondary motivation - but being strong for the sake of other people is a goal worth following to the end.