Hermione's First Dementor

This unnamed Dementor is a dementor of Azkaban, guarding the Hogwarts Grounds as of Spring 1993 on order from the Ministry of Magic.

Background
This Dementor, known to be a little under three metres tall, is one of the Dementors residing on Azkaban Island, employed by the Ministry of Magic for centuries as a guard for the Dark Wizards held there. Like the rest of the Dementors of Azkaban, it joined the forces of Tom Riddle during the British Wizarding War, but switched back its allegiance to the Ministry of Magic following Riddle's disappearance in 1981.

1992-1993
Following the escape of Sirius Black from Azkaban in the early months of 1993, the Ministry of Magic dispatched a legion of the Dementors of Azkaban to guard Hogwarts and apprehend Sirius Black should he attempt to infiltrate the Castle. The Dementor was among this group, and like the rest it boarded the Hogwarts Express on its way back to Hogwarts after the Easter Holidays.

Inside, it made its way to the compartment that happened to be inhabited by Hermione Granger. The Dementor telepathically explained its goal of finding Sirius Black, and, following Hermione's refusal to talk with it (which was due to the grumpiness itself caused by the Dementor's presence), it attempted to penetrate her mind. There, it saw that she actually did know the whereabouts of Black, and attempted to take her in custody; when she refused it got to work gaining complete control of her mind, consuming some of her happy memories while it was at it. Hermione tried to resist but her will was no match for the Dementor's powers.

However, its breaking of the girl's mind was interrupted by the Trolley Witch, on whom it quickly found it had no power. The Witch demanded to know what it was doing with Hermione Granger, and, when it answered that it was going to take her away, the Witch, enraged at the idea of somebody getting off the train before the station, called upon the magic of the train to take on a more fearsome, clawed form.

When she found her claws were useless against the invulnerable body of the Dementor, she resorted to channeling both her own magical power and the Hogwarts Express's as a whole to cast a Patronus that echoed throughout the train, and, taking the solid form of a steam engine, rammed the Dementor (along with all of its fellows) through a window and out of the train. As the Express had been stopped by the Dementor while atop a bridge overlooking some unknown body of water, the Dementor, who was too weak to fly, found itself slowly but inexorably falling down into the water.

Like its fellows, it later managed to extract itself from the water (its cloak soaked and partially frozen) and was force to glide on its own power the rest of the way to Hogwarts, where it began acting as a guard as planned.