Percy Ignatius Weasley, the third born child of the Weasley family, was put under the Imperius curse when he started working for the Ministry of Magic.
As prefect and head boy of Hogwarts school of Witchcraft and Wizardry, he helped make sure that the Gryffindor students were always in bed at curfew when Voldemort attempted to attack the school. He was always serious because he wouldn’t even understand a joke from his twin prankster brothers, Fred and George Weasley, even though they were saying it right in front of his face. He was always strict and perfectly behaved, but when he started working as a court scribe for the Ministry of Magic, his attitude changed greatly.
When he started working at the Ministry he became rude and aggressive to everyone. He started arguing with his parents alot, and he barely ever came home. Soon after, he broke up with his girlfriend, Penelope Clearwater, which made him even more miserable and angry. When Ron Weasley became a Hogwarts prefect, Percy Weasley sent Ron a letter saying that he should stay away from Harry Potter, because he could make Ron lose his position as a prefect.
The last thing that proves that Percy was put under mind control, is that he was determined to get his work at the Ministry done well, to please his boss, Mr. Bartemius Crouch. He became (too) fond of Mr. Crouch, and started to argue with everyone, telling them that Crouch was the best boss and best worker at the Ministry. People thought that because Mr. Crouch’s son, Barty Crouch Jr, was a death eater, that Mr. Crouch could have started controlling Percy Weasley’s mind.
In conclusion, Percy Weasley was put under an Imperius curse at the Ministry of Magic, for various reasons.