Darla is the 8-year-old niece of Philip Sherman, a dentist. Although she is not truly evil, and thus not a villain, she is rather rambunctious and often becomes overexcited when she receives pet fish, which ironically has frequently resulted in her accidentally killing them in her excitement.
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 in overreaction. Gill then fires himself out of the tank and lands in her hair, causing her to let go of Nemo bursting his bag. After Gill catapults Nemo into the air with a tooth mirror, Darla tries to grab him, but she misses and he lands in the sink and is sucked into the spiraling water going down the drain leading to the ocean. After Dr. Sherman returns Gill to the fish tank, Darla grabs the sink in attempt to retrieve Nemo, but she accidentally breaks it, causing it to harshly squirt water in her face. It is unknown what happened to her afterwards.
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 somewhat 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 actual harm, and apparently genuinely caring for Nemo (devastated when Nemo went down the drain).
The music that plays when Darla appears at her uncle's office is from 1960's Psycho during the shower scene.
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