Little Tooth Fairy
Baby Tooth is special Tooth fairy. She is one small fairy that is part of the numerous Toothiana's army that collects the teeth around the world, and she is one of the closes Jack's friends, and Jack has a protection steak when with her.
Their first encounter is at North pole, when Toothania checks his teeth, and while the others are nearer the Tooth fairy, this small one is nearest Jack, fascinated by him.
The second encounter is when Jack saves her before a Nightmare manages to capture her and so she doesn't share the fate of her sisters.
But I think she has one of the most important roles, when it comes to Jack's finding who he is and what is his purpose. When she is actually captured by Pitch, he uses her to get to Jack and his staff. But she is brave, and when Pitch doesn't hold his part of bargain, she bites him in the hand so she survives.
She and Jack are together in the cold, and the boy tries to warm her up, but can't do much because he is a winter spirit. Here is where she helps him, she helps him unlock the memories of his past from the tooth box, she is there when he remembers and she is the moral support that only the best friend can give. She is sad with him and she is happy with him, she gets excited with him, she is there when he needs her the most.
The fact that they are besties is also seen at the end of the movie, when Jack
throws the snowflake to the audience, we see her again on his shoulder.