Mertle is an incredibly spoiled little girl. She is the leader of a clique of girls who follow her every command. She strongly dislikes Lilo and calls her mean nicknames, such as "Weirdlo" and "Freaklo", as well as always getting her in trouble for what she did. 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 is 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, for continuity's sake). 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. Mertle is even cold enough to tease Lilo about being an orphan, which is Lilo's weak spot. She also blames Lilo for things that were her own fault, such as the time Mertle got an experiment pod on her charm bracelet and the experiment was a black hole and Mertle didn't listen to Lilo's warnings. When Holio's pod was activated, Mertle blamed Lilo for everything when she did not take 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 disfigured and ugly Earth animals.
In addition to being ignorant, Mertle also seems to have a bad memory, as she doesn't recognize Reuben, whom she had met in a few episodes, but then forgotten about. This is made worse by the fact that she can't read either, as shown during "Spike" when Mertle had Teresa to cheat for her during the Ohana-Rama. Then after Spike caused Teresa to lose her intelligence, Mertle couldn't answer a single question right.
In the TV series, it has been shown that Mertle's reason for mocking Lilo comes from her bigoted view of normality that causes her to disregard anyone different from her or anyone who disagrees with her. In Leroy & Stitch, it is hinted that Mertle's unseen father who she holds a great deal of respect for is the main influence of this view. Mertle has also been shown to be extremely dishonest as she had no qualms about cheating in contests and framing people for her wrong doings. However, it is shown that Mertle isn't totally heartless and dishonest, 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 have had some respect or admiration for Lilo's mother as she told her she'll never be a great dancer like her.
While it was not confirmed in the shows or movies, it is likely that Mertle's mother and father are separated or divorced. While it is never outright addressed, it is possible that Mertle's mother and father are separated or divorced. This is hinted at in Leroy & Stitch where when Mertle cruelly states to Lilo that Stitch "ran away and is never coming back," to which Yuki says "just like Mertle's dad, to which she snaps angrily for her to be quiet. This separation could be one of the reasons why she is a bully and has a poor attitude. Mertle could possibly hold respect for her father in order to bond with him and she adopted his views so she could see him more. The once-common (not anymore) view that divorce is not "normal" could be why Mertle is so mean to people who don't have traditional families, like Lilo. This would make her a hypocrite because she herself does not have the "traditional" nuclear family and likely tries to hide it from everyone, even her friends. This is again implied when Mertle mentions her father in Leroy & Stitch and Yuki is surprised that she has one. This would also make her similar to Lilo in the sense that the both have only a mother figure in their lives and have "broken" families.
Her poor attitude and bullying behavior could possibly and partially be attributed to her reaction to her parents' separation and her inability to see her father all the time. Her spoiled personality could also be from her mother spoiling her daughter to keep her happy and mask the struggles they endure as a family. Another likely reason that she is so mean to Lilo is that she is jealous of the love and happiness her rival has with her family, something she cannot get with her father away.
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, Teresa and Yuki. However, Mertle openly expresses her disliking toward the creature, calling it "the ugliest thing she has ever saw" and Stitch angrily kicks her off her trike and steals it.
Mertle serves as the main antagonist (or at least the closest thing to an antagonist) in Lilo & Stitch 2. 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 sporadically throughout. Lilo and Stitch end up interrupting a slumber party she was hosting (and did not invite Lilo to) while they chase down Experiment 221.
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 carrying Gigi while Stitch and his cousins battled the Leroys. In the end, Mertle and Gigi joined Lilo and her ohana in a huge family picture. Mertle still hates Lilo, but decides to be in the photo anyway because Gigi insisted.
A few comics for Lilo & Stitch were released before the original movie. The very first comic featured Mertle, but she was named "Jenny".
- Mertle's given name is a unique variant spelling of the female given name Myrtle, which is derived from the name of the plant Myrtus.
- Lilo often mentions Mertle by her full name. It is unknown why Lilo does this, considering that she doesn't seem to know anyone else named Mertle/Myrtle.
- In the Disney Adventures comic called Lilo's Luau, Mertle is called "Jenny".
- Mertle once "adopted" Hämsterviel, mistaking him for a talking gerbil.
- It is revealed in "Spike" that Mertle is illiterate and possibly has dyslexia.
- While it is never outright addressed, it is possible that Mertle's mother and father are separated or divorced. This is hinted at in Leroy & Stitch where when Mertle cruelly states to Lilo that Stitch "ran away and is never coming back," to which Yuki says, "Just like Mertle's dad," to which she snaps angrily for her to be quiet. This separation could be one of the reasons why she is a bully and has a poor attitude.
- Despite how mean and cruel Mertle is, Lilo still considers her a "friend" and will help when it is needed.