Darla is the niece of Philip Sherman, a dentist. She is not evil or malicious but rather rambunctious and having the overexcitement of having pet fish, which ironically has frequently resulted in her killing them by accident.
Gill and the other fishes in Sherman's fish tank describe Darla as a fish killer and that Nemo is supposed to be her new pet fish. The last fish she received, Chuckles, was killed by her because she wouldn't stop shaking the bag Chuckles was in. In the scene where Darla is about to take Nemo from her uncle, Nemo plays dead, but Darla thinks he is just sleeping and starts shaking the bag. Gill then fires himself out of the tank and lands in her hair, causing her to drop Nemo bursting his bag. After Gill catapults Nemo into the air, Darla tries to grab him, but she misses and he lands in the sink and is swallowed into the drain leading to the ocean. Darla then grabs the sink, but she accidentally breaks it, causing it to squirt water in her face.
Although Philip Sherman does not appear in this film, Darla's photo that her uncle has in his consulting office makes a cameo appearance in the Marine Life Institute when Dory meets Hank in the quarantine office. This may imply she has relatives working at the institute.
Darla, in a way, is very similar to Sid Phillips, as they are both naughty human children who serve as antagonists towards the main character's races (toys and fish, respectively), although while Sid is a genuinely nasty child (in the first Toy Story) who relishes in his sick experiments, Darla appears to be much more innocent and seemingly did not mean Chuckles and Nemo any real harm, and apparently genuinely caring for Nemo (evidenced by her calling out to Nemo when he went down the drain).
The music that plays when Darla appears at her uncle's office is the theme from 1960's Psycho.
Even though she is the secondary antagonist of the film, she is mostly seen in her picture rather than in person.
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