who are the main characters?

Oliver is the main character of the story. If I have to choose a second, it would likely be Fagin.

OLIVER IS THE MAIN CHARACTER

MAIN CHARACTER IS OLIVER.
AND THE IMPORTANT CHARACTERS ARE: NANCY, BILL SIKES, FAGIN, MR. BUMBLE AND MRS. CORNEY(MRS. BUMBLE)

MINOR CHARACTERS ARE: THE ARTFUL DODGER, NOAH CLAYPOLE, MR BROWNLOW, MONKS, MOSE MAYLIE, MRS MALIE, HARRY MAYLIE, TOBY CRACKIT, FAGIN'S GANG, MR. GRIMMING, AGNES FLEMING AND EDWIN LEEFORT.