Key: Last Name, First Name - Book first appears in. Short summary.
Davies, Roger - Book 3. Captain and Chaser of the Ravenclaw Quidditch team.
Delacour, Fleur - Book 4. An extremely beautiful girl (part-veela, in fact). Fleur was the Triwizard champion for Beauxbatons.
Delacour, Gabrielle - Book 4. Fleur's little sister.
Delaney-Podmore, Sir Patrick - Book 2. A decapitated ghost in charge of the Headless Hunt.
de Mimsy-Porpington, Sir Nicholas (Nearly Headless Nick) - Book 1. Resident ghost of Gryffindor Tower. Hit 49 times in the neck with a blunt axe, nearly severing his head.
Derrick - Book 3. Beater for the Slytherin Quidditch team.
Diggle, Dedalus - Book 1. Wizard who sent shooting stars off when Voldemort was defeated, and once bowed to Harry in a shop.
Diggory, Amos - Book 4. Cedric's father. Works for the ministry in the Department for the Regulation and Control of Magical Creatures.
Diggory, Cedric - Book 3. Prefect, Captain and Seeker of Hufflepuff Quidditch team. One of the Triwizard champions in Book 4.
Dimitrov - Book 4. Chaser for the Bulgarian Quidditch team.
Dippet, Professor Armando - Book 2. Headmaster of Hogwarts before Dumbledore.
Dobby - Book 2. The Malfoy's old house elf. Dobby always tries to help Harry Potter.
Dolohov, Antonin - Book 4. Wizard sent to Azkaban for supporting Lord Voldemort.
Dot - Book 4. Resident of Little Hangleton.
Dumbledore, Aberforth - Book 4. Professor Dumbledore's not-too-bright brother.
Dumbledore, Prof. Albus - Book 1. Headmaster of Hogwarts School. One of the greatest wizards of modern times.
Dursley, Dudley - Book 1. Harry's cousin. A spoiled brat if there ever was one.
Dursley, Marjorie - Book 1. Harry's "aunt". Always hated Harry, and always compared him to Dudley.
Dursley, Petunia - Harry's aunt and Lily's sister. Took care of Harry after his parents died.
Dursley, Vernon - Harry's uncle. Took care of Harry after his parents died (well, if you count stuffing him a cupboard taking care of him).