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Quote from the fairly odd parents wiki
"In "Tiny Timmy", it is said that her niceness never showed up to work inside her brain. In "Vicky Loses Her Icky", the cause of her evilness is attributed to an evil bug that crawled up her butt. In the episode "The Switch Glitch", she was actually nice as a five-year-old, but quickly turned bad when she wanted revenge on Timmy for being a mean babysitter. It has also been suggested in fanon that her sister Tootie being born was the cause of her disliking younger kids, although this has never been explicitly stated. "
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