This Girl is the protagonist of Roald Dahl's famous book The Magic Finger, but due to her actions, she is a protagonist villain. Her powers start the story and affect the main characters and completely disrupt their lives.
The Girl is unnamed, (common in many Roald Dahl characters) but she is eight years old and has supernatural powers which are concentrated in her forefinger. She is very temperamental and can explode into fits of anger, when this happens, her finger glows and sends electrical charges which hit the victim and mutate them depending on the issue which made her angry.
For instance, when her teacher Mrs Winter is angry at her for not being able to spell "cat" Mrs Winter calls her a stupid girl and the Girl gets really angry when she is told to stand in the corner, so she zaps Mrs Winter with her powers. Now under the Magic Finger, Mrs Winter is mutated into a human-cat hybrid, and the Girl outright expresses her evil act, saying "If you're wondering whether Mrs Winter is alright after this, the answer is no, and she never will be."
Later on, however, her main evil acts include cursing the entire Gregg family, (her neighbours) when they go hunting and kill a deer. This is bad, but it doesn't warrant the curse. The Girl zaps them all with her Magic Finger, including Mrs Gregg, when Mr Gregg ignores her lecture about hunting and tells her to piss off, and the next morning the Greggs all wake up with duck wings. They had been shooting ducks that day and killed sixteen, but had been followed home by four angry duck parents.
The curse goes onto have the Greggs homeless with the humanoid ducks taking over their house, only resolving itself when the Greggs vow never to kill any other animal like deer.
The Greggs do learn their lesson but the Girl doesn't seem to - she does rush over in a panicked state about the Greggs because she heard a duck answer the phone instead of Mr Gregg but she seems happy that her powers gave the Greggs a lesson in karma rather than doubting her abilities.