【How-to】Where is lady rainicorn from

Because Bubblegum is now “too young” to be legal ruler of the kingdom, her first creation, the Earl of Lemongrab (voiced by Justin Roiland), usurps the throne. … In the dungeon, Bubblegum realizes that she needs to return to her 18-year-old self, but she lacks the “candy biomass” necessary to do this.