It's sort of up to interpretation who counts as main characters, but here goes:

Harry Potter -- 12 year old wizard

Ron Weasley and Hermione Granger -- his best friends and fellow Gryffindors

Dobby the house elf -- a servant being who is central to this story even though he doesn't show up that much.

Rubeus Hagrid -- groundskeeper and Hogwarts and friend of Harry

Lucius and Draco Malfoy -- father and son enemies of Harry

Gilderoy Lockhart -- a posing blowhard who teaches Defense Against the Dark Arts

Severus Snape -- Potions teacher -- he and Harry hate each other

Albus Dumbledore -- Headmaster at Hogwarts

Ginny Weasley -- Ron's only sister.  Like Dobby, plays a big part in this even though she doesn't show up very often.