Factual error: In the beginning where Tooth is sending her fairies to various regions, she sends a few to Moscow, stating that there are "22 incisors and 18 premolars" that need collecting. Premolars are not baby teeth; they are permanent adult teeth. (00:06:10)
Factual error: When the Guardians start collecting the teeth, the skylines indicate that their first destination is Shanghai, but the billboard that the Tooth Fairy flies into is in a heavily Japanese visual style, from font weight to outlines and pastel colors. (00:37:05)
Answer: Jack is the guardian of fun. It's stated in the film.