Percy Weasley is a sixth year Hogwarts student and Ron's third oldest brother. He is a Gryffindor prefect, which means that he has the authority to discipline other students for breaking school rules. Percy is also utterly unlike the other Weasley children: he's really ambitious and he believes in following all of the rules as closely as possible. Ron comments, "'Course, he's really ambitious, Percy, he's got it all planned out…He wants to be Minister of Magic…" (4.146).
Percy's role in Chamber of Secrets is mostly minor, but there is a small mystery around him. Over the summer, Percy writes endless letters to someone, but he won't tell anyone who. When Ron and Harry take Polyjuice Potion to disguise themselves as Crabbe and Goyle, they bump into Percy near the Slytherin dungeons without an explanation. Why is he skulking mysteriously around the school?
When the mystery is revealed, though, it all seems trivial: it turns out that Percy has been dating a Ravenclaw prefect named Penelope Clearwater. Ginny caught them kissing and Percy swore her to secrecy. When Ginny tries to tell Harry and Ron about the Riddle diary, Percy interrupts her because he thinks she's telling them about his girlfriend. Again, Percy's role is minor and mechanical in this book. Still, we do think it foreshadows his unhealthy relationship to rules in Books 4 and 5 that he thinks he has to hide having a girlfriend so seriously. Why not just date her openly? Percy is so worried about being absolutely correct all the time that he causes himself a lot of unnecessary hardship.