Role in the series
Celestabellebethabelle makes her debut in the episode, "The Last Mabelcorn", where she is sought by Mabel, Candy, Wendy, and Grenda in hopes of using the unicorn's hair to protect the Mystery Shack from the demon Bill Cipher. When they meet her, Celestabellebethabelle claims she is the last of her kind and will only offer her hair to the purest of hearts. While Mabel expresses eager hopes that she is pure enough, Celestabellebethabelle disagrees, saying she can see deep inside her heart. This prompts Mabel to go on a quest of doing a thousand good deeds to meet Celestabellebethabelle's approval.
When the girls return to Celestabellebethabelle, the unicorn still states Mabel is impure, shattering Mabel's hopes of being good. To help their friend, Candy, Grenda, and Wendy get some fairy dust from the gnomes to knock out Celestabellebethabelle and attempt to cut a lock of her hair. However, Mabel stops them from doing so just as Celestabellebethabelle wakes up, mistaking Mabel as the perpetrator of the attempted act. She tells Mabel she will never be pure of heart. Unfortunately for Celestabellebethabelle, two other unicorns overhear her and reveal that the pure of heart story is a scam to get humans "get off [their] backs", and unicorns can not actually judge character. This causes Mabel to become engulfed in rage for Celestabellebethabelle's emotional manipulation. Celestabellebethabelle taunts Mabel, only to get punched back by her, leading to an all-out brawl between the two parties. After suffering a beat down from the girls, the unicorns give them a lock of Celestabellebethabelle's hair and some treasures to get rid of them.