I'm a sucker for a good redemption arc. I always make an effort to look for the good in a character, even when it's not easy. And to have the good inside them reach its full potential is always enjoyable for me. I like watching the usually morally ambiguous character turn into someone good. Seriously, if you want to get me hooked on a story, set up a redemption arc.
There are a few requirements, though, for a really good redemption arc. Now, I'm not going to list hard and fast rules, just some things I personally like to see.
1. It has to have a reason.
This obviously doesn't just apply to redemption arcs. Anything your character does have to have a reason or motivation behind it. That's just how real people operate. But you especially can't start off a drastic lifestyle change for a character without an adequate reason to prompt them towards it.
2. It has to be genuine.
This one is a little more complicated. You can have a character who deliberately appears to change in order to fool the others. There's nothing wrong with that. In fact, I'd say it can be a very clever twist. That's not what I mean by this requirement though. (Granted, the validity of this one depends on your character's personality and situations.) What I mean is that they can't simply flip-flop between sides. This only works if they've made a solid commitment. If they're just drifting towards good, then it doesn't apply. They still do have to have a reason for flipping sides repeatedly though. You may have to be careful to avoid them seeming wishy-washy.
3. Someone should believe their sincerity (hopefully).
But not necessarily without proof. And this doesn't necessarily have any bearing on how well the redemption arc is done. This is just a personal preference of my own. Whether or not this applies can also depend on what sort of life your freshly redeemed character led in the past. I just tend to think it's a little sad when a character is trying quite hard to be good and then not a single person shows any notice or support. (Someone can believe the character without giving trust, because that's totally understandable). I especially enjoy when the redeemed character eventually gets accepted into the group by the rest of the good guys (even if it takes a while).
I think I enjoy redemption arcs to much because, whether it regards fictional people or real ones, I firmly believe that as long as you're alive, it's not too late for you to turn your life around.
That's all for now! Odds are, there will probably be more on this at a later date, though. I tend to have a lot to say about redemption arcs.
Happy writing!