Baby Tooth is first seen during the attack on Tooth Palace where she was being chased by a Nightmare. Jack Frost managed to rescue her just in time and Tooth is happy to see that at least one of the fairies escaped. Following this, she hangs out with Jack mostly, following him around and hiding in his hood in times of danger. When the Guardians collect the teeth themselves, Baby Tooth mistakenly attacks a Tooth Mouse before Tooth tells her that the mouse is on their side. When Jack brings Sophie Bennett home, Baby Tooth accompanies him and tries to warn him not to follow the voice that is calling to him, knowing its dangerous. She still follows him into Pitch's Lair however and is captured by Pitch when she and Jack are separated. As a result of Baby Tooth being missing and Jack having his Tooth Box, the Guardians believe that he betrayed them and traded Baby Tooth for his Tooth Box, not even giving him the chance to explain. A heartbroken Jack flees to Antarctica where Pitch demands Jack trade his staff for Baby Tooth after Jack refuses to join him. Despite Baby Tooth telling him not to, Jack reluctantly trades his staff, but Pitch doesn't hold to the deal and Baby Tooth angrily stabs him with her beak. In response, Pitch throws Baby Tooth into a wall and she falls into an ice crevasse. Jack follows after his staff is broken and he's blasted down by Pitch. Baby Tooth quickly recovers, but Jack is unable to keep her warm due to his being the Spirit of Winter and naturally cold. Seeing how depressed Jack is, Baby Tooth activates his Tooth Box and signals him to open it. Seeing his memories restores Jack's confidence and he excitedly tells Baby Tooth that he had a family and a sister and saved her which he realizes is why he was chosen to be a Guardian. To his disappointment, Baby Tooth was unable to view the memories herself. Baby Tooth watches as Jack tries to repair his staff and is excited when he succeeds. Jack flies them out, telling Baby Tooth he owes her for giving him back his memories. Baby Tooth accompanies Jack as he tries to rescue her sisters, but they are unable to fly and Baby Tooth draws his attention to Pitch's globe which shows all the lights representing the children's belief going out. When Jack goes to help Jamie Bennett keep believing, Baby Tooth remains behind in Pitch's lair and watches the globe excitedly as Sandy restores the children of the world's belief. Baby Tooth and her sisters aid him by activating all of the stolen memory canisters before she returns to Jack and the Guardians in time to witness him becoming a Guardian. When Jack and Tooth hug, Baby Tooth interrupts them and flies with the Guardians back to North's workshop in the sleigh.
Like the other mini fairies she has green, blue, and purple feathers, translucent wings, and the nose of a hummingbird but bears certain traits that differentiate her from the other fairies. One of these certain traits are her eyes which are two different colors; her left eye is blue while her right eye is purple. She also has a distinct beauty mark under her right eye. Like Toothiana, she has a golden feather on top of her head while the other fairies only have blue or green ones. This might indicate why Jack Frost had given her the name Baby Tooth being as she is the only one besides Toothiana who shares this trait.
Baby Tooth is a caring, loving, and tough little fairy. She is also very brave and protective, seeing as she defended Tooth's Palace from Pitch before Jack saved her from the Nightmares. Since then Baby Tooth had helped Jack throughout his adventure and generally never left his side except when she aided Tooth in collecting some of the teeth in Europe. Notably, she followed him to drop Sophie Bennett off at home when she could've stayed with Tooth. Her close relationship with Jack sets her apart from the other fairies and some would argue stating that Baby Tooth is the only fairy who is not infatuated over Jack. However, this information isn't clear being as she showed signs of anger and jealousy when Toothiana embraced Jack near the end of the movie. Despite her tiny stature, Baby Tooth had proven herself useful throughout the movie and never backed down or left Jack's side even when danger approached exhibiting her undying loyalty towards others. When Pitch wanted to trade Jack's staff for her life, she clearly tried to signal Jack not to, even if it meant her own death. Jack did give up his staff to save her and when Pitch refused to keep the deal, she got so angry she stabbed him with her beak even though it was ineffective and resulted in him throwing her into an ice wall.
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related to Baby Tooth.
- When the Guardians were in Europe, Baby Tooth attacks a mouse and Tooth tries to calm her down. In Europe, the Mouse is described and used as a "Tooth Fairy."
- Baby Tooth has a gold feather, like Tooth, while all the other mini fairies have a blue feather sticking up from their foreheads.
- Jack Frost's sister had a distinctive beauty mark under her right eye, much like Baby Tooth's, which some fans take as a hint that Baby Tooth could be the reincarnation of Jack's sister.
- Baby Tooth had one purple eye and one blue eye, but in some shots Baby Tooth has 2 purple eyes.
- Baby Tooth and the other fairies are a part/extension of Tooth.
- In the books, there's a fairy that tells Katherine of the Guardians' plan to rescue her and the children from Pitch. This fairy may be thought as a book version of Baby Tooth.