"Once Upon a Time" takes our favorite Disney (and fairytale) characters and twists them into new stories. The norms state that heroes are heroes and villains are villains, but "Once Upon a Time" doesn’t follow these norms. Heroes can be turned and villains can be redeemed. The moral ambiguity of the show is what makes it so interesting, as every character can be seen in all viewpoints. (Be warned, there are a few spoilers here and there.)
The Evil Queen: Not as evil as you would think (now).
She started out good. Then her mother (The Queen of Hearts) messed with her emotions. She became evil -- to get her own happy ending like everyone else around her. As her storyline progressed, she ended up a hero. Her story proves that redemption is possible for anyone, no matter what they've done (like stealing hearts and killing parents).
Captain Hook: Yeah, he’s a good guy (for the most part).
Captain Hook was initially a bad guy, having an alliance with the Queen of Hearts. The alliance was broken, though, and now he's a hero. His redemption may have something to do with his love interest, Emma, but he has been redeemed nonetheless. Well, except for a little thing Emma did to him. Now, you should probably avoid the pirate at all costs.
Peter Pan: Stay away from that child at all costs.
Peter Pan is not the happy child who lives in the paradise that is Neverland like he is in the Disney movie. No, Peter Pan here is kind of a monster. He was an old man who left his son behind so that he could be forever young. He left everything behind for youth. PSA: Never fly with Peter Pan.
Mulan: As strong and independent as ever while being into girls.
Mulan is a warrior, obviously, but "Once" changes her story around a little. Mulan holds romantic feelings towards Princess Aurora and proves that something like sexuality doesn't prevent a person from being strong. She may be into Princess Aurora, but she is known as one of strongest warriors in all the lands.
Red Riding Hood: She also happens to be the Wolf.
Red's red hood actually prevents her from becoming the Wolf. The Wolf is evil by nature, but Red proves that she wants to do good and is appalled by the horror. She even kills her own mother in defense of her friend, Snow White.
Prince Charming: He used to actually be a shepherd.
His twin was the actual prince, but after his brother died, he was taken by his brother's adoptive father. He shows that you don't have to be born into royalty to be a good leader or a great leader. You can be a shepherd, even.
Ursula: She used magic to escape a father who exploited her.
Ursula is a villain, sure, but what caused her to become one holds reason. Her father, Poseidon, exploited her singing voice when she was a mermaid. When she finally had enough, she tried to use magic to escape him. The magic was supposed to give her legs, but instead she turned into the half-octopus sea witch, and as Rumple says, "Magic always comes with a price." (She did escape him, though.)
Rumplestiltskin: He also happens to be the Beast.
Speaking of the Beast, Rumplestiltskin has always been a complicated character on the show. He turns to dark magic to save his son: good. He exchanges help to a man ... for the man's daughter: bad. He's a manipulative man, but he does have a good heart in him. He even sacrificed himself to save the whole town from Peter Pan. (He's still alive, by the way.)