Libby Flanders is the second of Alex Mack's rivals at school, after Jessica. In the episode False Alarms, Libby tries to get out of taking a test by pulling the fire alarm and convinces Alex to take the blame for it in exchange for being allowed to hang with her. Alex initially goes through with it due to being jealous of Libby's popularity, but then realizes that Libby really cares nothing for her. When Libby attempts to pull the fire alarm again a week later, Alex uses her powers to stick Libby's feet to the ground and leave her holding the lever as the teachers arrive, exposing her as the culprit.
Though she mentions that she will return and make Alex pay for her humiliation, Libby never appears again (save for a small cameo in the beginning of the series finale). By Season 2, Kelly Phillips replaces Libby and Jessica as Alex's full-time school rival.