House: Gryffindor, 7th Year; Head Boy
"P" stands for "Pompous" here. Percy is like a male version of Hermione in some ways – he's an over-achiever, he abides by the rules, he's (overly) proud of his Head Boy status, and is a perpetual stick in the mud. These two have trouble having fun, too.
As Percy hoped to enter the Ministry of Magic, he needed top grades. He was becoming increasingly edgy, and gave very severe punishments to anybody who disturbed the quiet of the common room in the evenings. In fact, the only person who seemed more anxious than Percy was Hermione. (16.1.2)
Unlike Hermione, though, Percy is extremely arrogant and conceited; for all her faults, Hermione isn't smug. Percy also isn't down with breaking the rules even if the situation calls for it. He's a bit of a running joke in his own family. The twins especially have fun mocking Percy. Percy could be a cool guy if he'd just loosen up some and get down off his high horse, but as it is he's alternately amusing and obnoxious. We don't really blame the twins for wanting to lock him up in a pyramid in Egypt.