Zoro is the first member of the Straw Hat Pirates. His dream is to become the world’s greatest swordsman.
Zoro laid down his life in defense of Merry, though he probably did not even know her name at that time. After these events, Zoro decided never to sacrifice innocent lives again and always fight fair and square without resorting to underhanded tricks or relying on other people’s strength except for his own.
To match with Luffy’s progress, Zoro trained himself non-stop while maintaining his daily duties of gathering food and water by hunting local wildlife (presumably for himself as well). He became more powerful than before, able to defeat entire bands of pirates single-handedly.
He also defeated multiple enemies who were several times stronger than he was, among them the Shichibukai Dracule Mihawk.
Through his journey in search of Luffy, Zoro has discovered an incredible will to live after escaping death at the hands of Mihawk’s Kokutei Roseo when he could have easily succumbed due to his severe injuries (although this may be attributed to him wanting to fulfill his dreams).
This undying conviction is most often displayed during battle; no matter how grievous his injuries in battle are, as long as there is even a single strand of life left in him, he would not succumb until he has fulfilled his purpose (defeat the enemy standing before him). This very act of refusing to give up causes many observers and allies alike to label Zoro as insane.
Zoro is extremely strong-willed, having the ability to overcome nearly any form of adversity that comes his way. This trait has not only strengthened his willpower, but also helps him survive the direst of situations at times where most others would have given up long ago. His mantra seems to be about fighting with all of my might, and I’ll never surrender until I’ve defeated my enemy. He will do whatever it takes if he knows he can gain something by doing so or find personal satisfaction or pleasure in doing so.