She was a relic of the show's original concept where there was no magic and Star was just an autistic loli trapped in her own fantasy world.
Nefcy changed it so Star actually was a magical princess instead of a LARPer, and tried to remodel the cast she had already planned around the original concept to the new one, but some didn't really work. For example: this guy. He obviously would have been the jerkass who picked on Marco in the original version, but in this version Marco can beat up monsters, so this guy wasn't really any believable threat to him.
Brittney was another character who didn't "fit" into the new version. She was basically like Gem from the Sabrina the Teenage Witch cartoon or the Krust Cousins: their main "power" over the main character was their greater wealth and popularity. And she would have been an actual antagonist back when Star was just some friendless sperg.
But Star in the show is just as if not more wealthy and popular than Brittney, so the latter didn't really serve as a much of a threat to the former.