Barty Crouch Jr.

Bartemius "Barty" Crouch, Jr. (gone undercover as Dark Wizard hunter Alastor "Mad-Eye" Moody) is the main antagonist of the fourth Harry Potter book, Harry Potter and the Goblet of Fire and its film adaptation. He is a Dark Wizard and an infamous Death Eater.

He spends most of the story posing as the friendly teacher Alastor "Mad-Eye" Moody and became the Defense Against the Dark Arts (D.A.D.A.) teacher at Hogwarts School for Witchcraft and Wizardry during the Triwizard Tournament, hosted by Hogwarts. He was sent by Lord Voldemort, the most powerful of all Dark Wizards and leader of the Death Eaters, to fix the Triwizard Tournament so that Harry Potter wins the tournament and use his blood to give Voldemort a new body.

In the film, he was portrayed by David Tennant.

History
In 1981, after Lord Voldemort lost his powers after trying to kill Harry Potter, Barty (then 19) along with Bellatrix Lestrange and other Death Eaters tortured Frank and Alice Longbottom into insanity using the Unforgivable Curse Cruciatus to find any information on Voldemort's whereabouts. Presented before the Wizengamot they were sentenced to Azkaban. Barty openly pleaded with his father Bartemius "Barty" Crouch Sr. for mercy, but the long-strained relationship between them and Crouch Senior's own standing in the Ministry meant that this was impossible. According to Sirius Black, Barty was screaming inside his cell for his mother for the first few days but then quieted down from the extreme depressive influence of the Dementors, as did many other prisoners.

One year later, his mother's health had deteriorated drastically from the stress of her son's imprisonment. The couple were allowed a deathbed visit of their son due to Bartemius Sr.'s high status in the Ministry. She persuaded her husband to help smuggle their son out of Azkaban by swapping him for his mother. Using Polyjuice Potion, and undetected by the blind Dementors, Mrs. Crouch took the place of her son. She died a short time later and was buried outside the fortress under the guise of her son.

Bartemius Sr. was overcome with grief and guilt and opted to control him via the Imperius Curse to protect his son by keeping his existence secret from the public, and to keep the son from continuing the search for Voldemort. He also forced him to hide under an Invisibility cloak at all times. This was more directed at honoring his beloved wife's last wish, and while some small affection for his son came through, the methods he used to do this arguably drove Junior further into madness.

Bertha Jorkins, a Ministry of Magic employee, accidentally discovered the truth when she came unannounced to the Crouch home and discovered their secret. Bartemius Sr. rectified this with a Memory Charm so powerful that she suffered permanent brain damage. Years later, Peter Pettigrew ran into Bertha while searching for Voldemort. In the hopes that she would prove useful to him, Peter tricked her into going with him. She ended up proving very useful to Voldemort; not only did he extract the whereabouts of his loyal servant, but also gathered information on the Triwizard Tournament, which was to be held at Hogwarts. Voldemort murdered Bertha, and the Ministry deemed her "missing" for months.

Trivia

 * Barty Crouch Jr. is similar to the shadow version of Professor Avalon in Winx Club: Both impersonate a character who was going to teach at the magic school but was imprisoned before they could. Both get close to the main character, and both ultimately help their masters, Lord Voldemort and Lord Darkar, achieve their goal (for Voldemort, returning to power, and for Darkar, obtaining Alfea's Codex and capturing Bloom for his ritual). As Harry Potter is one of the main inspirations of Winx Club, Barty and Moody were likely the inspiration for Avalon and his impostor.
 * Crouch Jr. is very similar to the secondary Bigger Bad of the franchise; Gellert Grindelwald as the latter disguised himself as Percival Graves and so does Jr. as he disguised himself as Moody.
 * The film version of Crouch has been described by some as more evil than his book version. In the film, Barty is a fanatical follower of Voldemort out of sadism, when in the book he was neglected and abused by his father and that was a reason he was seeking a father substitute in Voldemort. That said, Barty's father is more likable and sympathetic in the film partly due to Winky's omission.
 * Barty Crouch Jr. is widely regarded as a favorite among the Death Eaters by fans of the series despite making just one appearance.