To me, one of the major things that make Zoro as a character is his sense of duty on his path to the top. There's an element of selflessness to his character, rooted since Kuina passed away. Despite possessing from the gate possibly the most single-minded goal of the Straw Hats, it is carried out on behalf of several characters. First his late friend and rival Kuina, then Luffy and the Straw Hats by promising to never lose, and now Wano where he wishes to liberate the people after witnessing their suffering. I like to interpret his proclaims to become the 'King of Hell' as taking back the Country considered hell on earth.
He does not consider his ambition worth following at this point if it cannot even defend the ones he has personally vowed to fight for, such as Luffy, the crew, and Kuina through the Wado Ichimonji. This is because the strength he's gain has no meaning if it is not directed with a purpose. It is that lesson he's learned from Koshiro, almost instinctively as he's matured.
He's also a bit of a breath of fresh air. Too often than not, these strong silent types are loners who act on their own, and are grim and always serious. But Zoro isn't above goofing around when it's fun times. Loves a good party and drinking. Can be as clueless as his captain in some situations that can't be solved through his expertise. This combatant with the sharpest of instincts somehow doesn't know his way around a one story house if left to his own devices, which hilarious irony to a degree and endears us to his character in lighter moments. And he values the bonds he's formed. Reminding you that under all that cool edge is a person.
Extending back to the first point, a great quality of Zoro, which you can also interpret as a character flaw, is his unwillingness to share how he feels. Perhaps not to show weakness or to not make his allies worry for him. He refuses to allow himself to be a burden, which almost always stems from those he's trying to protect or fighting for. Rika being the first in the story, who he saved without hesitation, and even offered to starve himself if it meant her assured safety, and refused to let her know he was hungry (a very samurai like trait).