Stella is the ringmaster of a Pokémon circus which once camped on the outskirts of Pallet Town. The star of the show was a Mr. Mime, but one day it stopped listening to Stella and just sat in its trailer all day eating, eventually becoming overweight.

Although she appears sweet outside of the circus, her training methods prove to be very tough. When Ash attempted to act as Mr. Mime for the circus, he was subjected to being whipped and yelled at, remarking that he wasn't surprised that Mr. Mime quit if that was how it was treated.

Upon realizing that she pushed her Pokémon too hard, Stella apologized to Mr. Mime. It accepted her apology, and later rejoined her at the circus.

(source: Bulbapedia)