Mertle is an incredibly spoiled little girl. She is the leader of a clique of girls who follow her every command. She has a strong dislike of Lilo and refers to her as "Weirdlo" and "Freaklo" for most of the franchise. She is shown to be unforgiving and holds grudges against people, even when they apologize, as shown when Lilo apologizes for tackling and biting her earlier, she does not forgive her and only says rudely "Apology not accepted."
It's shown in the TV series that her "friends" aren't too fond of Mertle's mean and bossy attitude, implying that, despite the fact that they hang out with her, they don't actually like her, in fact, they sometimes leave her (but then, come back eventually). Mertle takes great pleasure in putting down, insulting, bullying and making fun of Lilo. Mertle also tends not to take responsibility for her actions; she blamed Lilo for beating her up in hula class, even though she was being cruel to Lilo, which provoked her into beating her up.
She also blames Lilo for things that weren't her fault such as the time, Mertle got an experiment pod on her charm braclet and the experiment was a blackhole and Mertle didn't listen to Lilo's warnings, and when Holio's pod was activated, Mertle blamed Lilo for everything, when she was the one not taking Lilo seriously. Just like the other humans, she is too ignorant to tell Stitch, Reuben and the other experiments are aliens and usually mistakes them for disconfigured and uglified Earth animals.
Mertle's dad may have abandoned her at one point, which could be why she's mean all the time. However, it is shown that Mertle isn't totally heartless, in the episode Yapper, she cheated to win the dog show, but then gave Lilo the trophy, because Lilo played fair and she also loves her dog, Gigi. It has also been hinted how she might've had some respect or admiration for Lilo's mother as she told her she'll never be a great dancer like her.
Mertle is a young girl attending the local hula class alongside Lilo, Elena, Teresa and Yuki. She gets into a fight with Lilo during one of their practices. After Lilo adopts a "dog" named Stitch, Lilo introduces her pet to Mertle, Elena, Teresa and Yuki. However, Mertle openly expresses her disliking toward the creature, calling it "the ugliest thing she has ever saw" and Stitch kicks her off her trike and steals it.
In the official sequel, Mertle serves as the main (yet minor) antagonist (or at least the closest thing to an antagonist. She enters a hula competition against Lilo, which prompts the latter to practice really hard, ignoring Stitch (who is in serious danger of dying) and putting a strain on their friendship.
Mertle is a minor character in the film, appearing back and forth throughout the picture.
She became a recurring character in Lilo & Stitch: The Series. Throughout the series, it is established that she is from a relatively wealthy family and is a spoiled brat who lives with her mother. In one of the episodes, it is revealed that she keeps Gigi (Experiment 007) as a pet, but does not realize she is an experiment. Sometimes, Mertle is associated with Hämsterviel, but she's also blind to the fact that he's evil and using her.
In the episode "Finder", she mistakes Dr. Hämsterviel for a gerbil and keeps him as a pet for a short time. In "Shush", when her friends break up with her after Lilo wrongly thought she hated them, she teams up with Dr. Hämsterviel, who apparently turns her into an android in order to catch all the other experiments in place of Gantu. However, she was rescued by Lilo and Stitch, who took her to Jumba to de-robot her. Lilo tries to be friends with Mertle in several episodes, but this usually just results in even more social rejection.
She is rejected by her friends in "Tank" and teams up with Stitch to capture the experiment. They fail and are captured but manage to escape in Stitch's dune buggy while Gantu gets the experiment. She initially tries to tell her friends about it, but stops and becomes friends with them again.
In the final film, Hämsterviel plotted to rule the galaxy using an army of Stitch clones named Leroy. He orders Leroy to kidnap all of the other 624 experiments, including Gigi. When Gigi was captured, Mertle was taken as well. At the final battle, Mertle ran in fear while Stitch and his cousins battled the Leroys. In the end, Mertle and Gigi joined Lilo and her ohana in a huge family picture.
A few comics for "Lilo & Stitch" were released before the movie. The very first "Lilo" comic featured Mertle, but she was named "Jenny."