Mertle is established as Lilo's ex-friend at the beginning of Lilo & Stitch, where she is introduced as a popular girl and the leader of a clique composed of herself, Elena, Yuki, and Teresa in the hula halau. Mertle calls Lilo "Weirdlo" and refuses to include her in the girls' activities, in which Lilo responds (quite violently) by hitting her, pulling her hair, and biting her. Mertle and the other girls express fear and disgust of Lilo and her things, such as Scrump the Doll and Stitch. Mertle takes great pleasure in putting down, insulting, bullying, and making fun of Lilo.
However, Mertle has an attitude adjustment when she discovers that Gigi likes the other "cousins" and can talk. Lilo still considers Mertle and her posse to be her friends.
Mertle claims she's unable to whistle like Lilo can, and is more of a twice-removed part of her ohana.
In "Spooky", it is revealed that her worst fear is seeing Lilo without her head.