1. The ability to prove the doubters wrong. I love it when the underdog wins the day. Whenever I read of a character being "written off", I am on the look out for that character turning out to be the hero (usually) or the villain (sometimes, and inevitably reacting against being written off! I do have some sneaking sympathy here. You can see why they would react that way at least.).
2. The ability to stick with the right path, no matter what. The ultimate example of this for me is both Frodo and Sam in The Lord of the Rings. (Sam was just as determined to do right as Frodo was, even though Sam was doing this for his friend, rather than for the "cause" directly).
3. Being the best friend/sidekick the hero didn't know they needed until undergoing the quest. If awards were being given out for this, it would have to go to Sam Gamgee. Never ever overlook the sidekick. They're in the story for a good reason and it is almost always a pivotal moment the hero needs.
4. Honest characters. I love those moments when, usually the sidekick, gives the hero/heroine a verbal bashing for missing something important or taking others for granted or for becoming arrogant etc. The lead characters do need others who can rein them in - nobody gets it right in life all the time so why should they in fiction? Just as we need others to tell us "hang on a moment there", so do characters need other characters to tell them when they're at risk of going off track.