I'm surprised nobody's mentioned Ace. He was known for his bordering on stupid habit of never backing down from anybody, even if it meant death. He had this resolve as a kid and would be willing to die fighting grown ass pirates because he didn't want to run away "leaving anything behind". Then we have him in a weakened state trying to fight Akainu despite being miles out of his league. Ace backed down from nothing and protected anything dear to him and did so since day 1, even as a little 10 year old saving Luffy against Bluejam's crew etc..
That said, Luffy's right up there with him along with Zoro, Sanji, Roger, WB and a few others I'm probably forgetting. One piece is full of characters with heart.