Ministry: Department of International Magical Cooperation
Percy Weasley, the Weasleys' third son, has always been a slightly stuck-up and self-absorbed guy. He's definitely the odd one out in the Weasley family: he believes in rules and respectability above all else. Now that Percy has graduated from Hogwarts and is working for the Ministry of Magic under none other than rule-abiding (for the most part) Mr. Crouch, he's got a deeply puffed up sense of his own importance. This sense of importance is unshakeable, even when it becomes embarrassingly clear that Mr. Crouch thinks Percy's name is "Weatherby," and not "Weasley."
Percy's self-importance only gets more intense when Mr. Crouch takes a leave of absence from the Ministry because of a mysterious illness. Percy becomes Mr. Crouch's mouthpiece to the world. And he absolutely loves this position, stating condescendingly about his missing boss, "no, poor man, he's having a well-earned, quiet Christmas. I'm just glad he knew he had someone he could rely on to take his place" (23.85). Right.
If Percy had been slightly less eager to take power from Mr. Crouch, maybe he would have noticed that it was a bit odd for Mr. Crouch to take days off. After all, as Sirius comments, "If he's ever taken a day off work because of illness before this, I'll eat Buckbeak" (27.139) (you're hilarious, Sirius!). Percy's character flaws make it possible for Voldemort to keep Mr. Crouch under his control.
Percy's extreme ambition also seems to be a reaction to his father's mediocre Ministry career. Percy is desperate for the kind of social stability that Mr. Weasley has never given his family. Ron asks an extremely important question, one that gets taken up in the later books: "I don't know [...] If [Percy] thought we were standing in the way of his career ... Percy's really ambitious, you know ..." (27.209). Percy admires Mr. Crouch hugely, which makes us wonder if, later in the series, Percy is going to have to choose between his family and his career the way Mr. Crouch did.