Jillian's bio is available in the minor character docket and is as follows:

Jillian is Pierce's little sister and a give-no-shit take-no-shit lady Bart Simpson. She reoccurs for three episodes in the series and joins Shanelle and Jett in the season one regatta, "Scream at the Sea." She's a troublemaker with an affinity for guns, holds Pierce accountable for his actions and will probably save the world from the rest of the cast.