He has functional mouse ears and a tail and suffers from insomnia as a result of constantly drinking coffee out of paranoia at the thought of being eaten by Boris. He used to work for the Hatter family, but ran away prior to the beginning of the series due to being terrorized by Boris and the twins. He adores Nightmare because he helps him sleep and is often worried about his health. Pierce is terrified of Boris who often tries to eat him and constantly makes him feel like his life is in danger. However, he likes Peter and Elliot because he doesn't feel threatened around rabbits.
Source from Wikipeida