Percy is the third oldest Weasley child. He is two years older than Fred and George. Percy has always been kind of a goody-two-shoes know-it-all, but basically a good egg. He and Harry overlapped at Hogwarts for three years, and they always got along fairly well. In Book 4, when Percy starts working for the Ministry of Magic, he gets incredibly puffed up and pompous, but it all seems in good fun: Percy worships his boss, who calls him "Weatherby," Fred and George laugh at him, and everything seems fine.
Then comes Book 5, when we scarcely see Percy at all. The reason that we don't see Percy much is because he has basically disowned the Weasley family and thrown in his lot with Cornelius Fudge. Percy thinks that Fudge is the way for him to establish a real career for himself. He wants to do better (career-wise) than his father, so Percy willingly turns his back on Professor Dumbledore and joins with Fudge and Professor Umbridge. Whenever Fudge appears in Book 5 – trying Harry for the underage use of magic or attempting to arrest Dumbledore for treason – Percy is there to support Fudge and take notes on his every move.
Percy's estrangement from the family is really hard on the Weasleys – especially on Mrs. Weasley, who is completely devoted to her children. But it doesn't really come home to Ron until he receives a long letter from Percy. Percy hears that Ron has been made a prefect. Percy himself was a prefect and then Head Boy before he left Hogwarts. Thinking that Ron has decided to follow in Percy's footsteps, Percy writes to Ron advising him to ditch Harry as a friend. After all, being "tarred with the same brush as Potter [...] could be very damaging to [Ron's] future prospects" (14.192). Percy also tells Ron that he expects Professor Dumbledore will be removed from Hogwarts soon, so Ron should cozy up to Professor Umbridge. Ron is so horrified by Percy's self-important, conniving advice that he tears the letter up.