Auron's character was rather static. He went through one minor character revelation that changed him slightly. Other than that, he is the same character throughout the whole game, only getting character development in hindsight and backstory, and even that is only slight compared to the scope of the game and what other characters go through.

That being said, Auron's character was rather awesome. His is the type of character that doesn't need character development, he's already developed, and Square did an excellent job portraying that aspect. The type of character that he is is lost for the most part on our generation. Doing something on this scope for the pure sake of brotherhood and a man's dieing wish is a concept that isn't very heard of in this day and age. His unwavering, and thus static, character is actually the definitive point that makes character so great, and if he didn't have that aspect his character wouldn't make sense. As easy as it sounds storywise, it is actually quite difficult to pull off a static character and keep him interesting through the whole story, but Square did it rather well.

I have a soft spot for Lulu, and if you combine X and X-2, I also have one for Yuna, but in the scope of just FFX, Auron is definately one of the two best characters in the game, if not the best. Best in the series, I don't know, he's up against some pretty good contenders. That would be difficult to assess.