Guts from Berserk is a fairly dark example. Despite repeatedly taking injuries that would kill anyone who wasn't Made of Iron he never stops fighting. Griffith is an antagonistic example, considering he manages to move despite having the tendons in his arms and legs severed and is willing to sacrifice every single person that he cares about in order to fulfill his dream.
Hareta from Pokémon: Diamond and Pearl Adventure. No matter how much you beat him, he'll come back with a grin and a renewed determination to train harder and fight better. And no matter how many threats you make against him, his friends, or any Pokémon, he'll be doubly determined to stop you.