Dementor who searched Harry Potter's compartment

This unidentified Dementor was one of the Guardians of Azkaban in the 1990's.

Origins
It is possible that the Dementor was one of the original Dementors found by the Aurors when they investigated Azkaban following the death of Ekrizdis, as Dementors were immortal. It is also possible that this particular Dementor was "younger" and had grown from the despair inherent to Azkaban at some point before 1993. If he was old enough to have done so, the Dementor doubtless followed his kin during the First Wizarding War, deserting his duty as Guardian of Azkaban and joining forces with the Dark Lord and his Death Eaters. Also like his fellows, he would have then reverted his allegiance to the Ministry of Magic in 1981 following Voldemort's first defeat.

1993-1994
Hogwarts Express (Adam Brockbank's Concept Artwork for HP3 movie).jpgIn September 1993, the Dementor was among the Dementors of Azkaban dispatched to Hogwarts by the Ministry of Magic to guard it against a possible attack by Sirius Black. Like the others, he boarded the Hogwarts Express part-way through its trip and began searching it for signs of Black.

Soon, the Dementor found his way to the compartment housing Harry Potter, Ronald Weasley, Hermione Granger and a then-slumbering Professor Remus Lupin. He slid the compartment's door aside with a wave of its moist, dead-looking scabby hand, and stood in the doorway, illuminated by the light of a now-awake Lupin's Bluebell Flames. Realizing his hand was visible, the Dementor quickly withdrew back into the folds of his black cloak.

After this split-second of indecision, the Dementor began using his magical breath to suck out the happiness and warmth out of the compartment. This had a particularly bad effect on Harry Potter, on whom the Dementor seemed to focus especially (which Remus Lupin attributed to Harry being easier prey because of all the terrible things he had gone through). Harry fainted and fell out of his seat, prompting Lupin to step forward and tell the Dementor that Sirius Black was obviously not in their compartment, and that he must leave. The Dementor refused to comply, apparently eager to keep feeding on Harry Potter; realizing he would get nowhere by bargaining with him, Lupin cast a non-coroporeal Patronus Charm to drive the Dementor away, and the Dementor withdrew. He soon joined the rest of his fellows and left the train.

Throughout the rest of the year, the Dementor was presumably at Hogwarts as intended, supposedly guarding it from Sirius Black. It is unknown if he was present when some of the Dementors attacked Harry Potter on the Quidditch pitch, although it is nearly certain he was among the Dementors who crowded around Sirius Black and Harry Potter, eager to suck out his soul, near the end of the year, only to be driven away by Harry Potter's exceptionally strong Patronus.

1996-1998
During the Second Wizarding War, the Dementor, along with all his fellows, switched his allegiance back to Lord Voldemort and deserted his duties in Azkaban to fight alongside him. It is likely that he was present at the Battle of Hogwarts, where Voldemort had pooled his entire army (Dementors included) in an attempt to finally destroy his opponents and take Hogwarts Castle.

Abilities and skills

 * Wandless magic: The Dementor appeared able to slide a compartment door on the Hogwarts Express through a mere wave of his hand.
 * Dementor's Kiss: Like all Dementors, the Dementor could presumably perform the dreaded Kiss on a human being, sucking out their soul and leaving their body an empty shell.

Behind the scenes

 * In the film adaptation of Harry Potter and the Prisoner of Azkaban, the Dementor displays an aptitude for telekinesis, sliding the compartment door open without touching it. In the books, it is mentioned that "the compartment door slid open", leaving it ambiguous whether the Dementor simply pulled it aside silently or used magic.
 * Due to how hard it is to tell Dementors apart, it is unknown if this particular Dementor was present in any subsequent incidents involving limited groups of Dementors — such as the attack on Harry Potter and Dudley Dursley in 1995 or Dolores Umbridge's guard in her courtroom.