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.