Barty Crouch, Jr.
Affiliation: Death Eater
Barty Crouch, Jr. is the Death Eater who impersonates Mad-Eye Moody (using Polyjuice Potion) for an entire school year. He has been keeping the real Mad-Eye Moody in a trunk, and it's been the Death Eater Barty Crouch, Jr. who has been befriending Harry Potter and turning Draco into a ferret. He's been doing this all along with the motive of getting Harry to the Triwizard Cup at the right time to be transported to the Little Hangleton graveyard and Voldemort's waiting arms. Barty Crouch, Jr. agrees with the real Moody that, "if there's one thing [he hates] more than any other, it's a Death Eater who walked free" (35.68). But Barty Crouch, Jr.'s reasons are completely different: he hates anyone who has deserted his one and only master, Voldemort.
Barty Crouch, Jr.'s problems seem to stem from his dad. His father is a powerful member of the Ministry of Magic, who's so attached to his own job that he doesn't seem to regret eventually condemning his own son to Azkaban Prison for being a Death Eater. Barty Crouch, Sr. is a rigid, self-righteous man who must have made young Barty Jr.'s life difficult and lonely. Barty Crouch, Jr. states that he wants to be "closer than a son" (35.81) to Voldemort: he's pretty obviously looking for a father replacement.
It's interesting that J.K. Rowling has commented on the father figure theme more generally in Harry Potter:
As I look back over the five published books [...] I realize that it's a kind of litany of bad fathers. That's where evil seems to flourish, in places where people didn't get good fathering. (source)
It's not just Voldemort who suffers from a bad father: there's also Severus Snape (see Harry Potter and the Deathly Hallows), Sirius Black (see Harry Potter and the Order of the Phoenix), and, of course, Barty Crouch, Jr. himself. Barty Crouch, Jr. is an interesting example because we know that his mother loved him enough to substitute herself for him in Azkaban (since the Dementors, the Azkaban guards, are all blind). Barty Jr. also had the devoted love of Winky, the Crouch family house-elf. But neither of these influences were enough to balance the rejection he clearly feels from his father. For all of these guys, it comes down to their dads in the end.