Star Wars has a long, proud tradition of bad guys—or, at least, disinterested neutrals—who see the light and sign on for Team Hero.
But we've never seen anyone as deep in the system as Finn.
Finn the Fearful
We first see him as a stormtrooper, getting ready to wipe out an innocent village in the name of Scary Guy Needs a Map. Turns out, he's not down with the program, highlighted by one of his fellow stormtroopers slapping blood all over his helmet. That prompts a moment of clarity, and the former faceless, nameless drone in the First Order machine decides that he could do much better somewhere else.
His early actions are governed largely by self-interest, however, and while he's not a jerk about it, he doesn't make bones about whom he looks out for. Fear seems to be his primary motivation: fear of the First Order and what they can do to him:
FINN: You don't know the First Order like I do. They'll slaughter us. We all need to run.
As we all know from our Yoda readings, fear is the path to the dark side, and it's what Finn needs to conquer if he really wants to get with the program. He first understands the consequences of acting like a Scaredy McWetpants when Rey is abducted, and he's powerless to prevent it:
FINN: He took her! Did you see that? He took her. She's gone!
That shakes some of the cobwebs out of his head and gives him the focus he needs to really leave his stormtrooper past behind him and become a rebel. Rey was his friend, and he let her down. Now, he needs to make that right by putting her ahead of himself for the first time, showing him that safety only matters if you've got some BFFs to share it with.
He gets a little boost in that department from Poe, who he thought had died in the TIE fighter crash but who pops up alive and well at the Resistance Base. He also lets Finn know what he did by saving Rey and BB-8 on Jakku and how important his actions might be to the fate of the galaxy:
POE: You completed my mission, Finn.
It's definitely enough to make him sit up and take notice. So he goes after Rey, who's being held captive on Starkiller Base, and even defends her against Kylo Ren in the film's darkest moments. But it's always in the darkness that we learn who we really are, and Finn, though still frightened, isn't going to let that beat him.
Decisions like that are the stuff that heroes are made of, and we're pretty sure that this anonymous little stormtrooper is going to have a whole lot more to say about the fate of the galaxy as the story goes on.