Professor Moriarty is a master criminal and Sherlock Holmes' Arch-Rival. They enjoy matching wits against each other but Sherlock recognizes that Moriarty is extremely dangerous. Moriarty resists killing Sherlock on at least one occasion because he likes the challenge of Sherlock trying to catch him. Moriarty has connections throughout the criminal underworld, and calls himself a "consulting criminal." There doesn't seem to be anyone he can't pay off to achieve his plans. He is ruthless and cruel and enjoys toying with people. He doesn't hesitate to use innocent people in his plans as pawns, killing them to entertain himself. When he first meets Sherlock he uses a disguise that fools the great detective, which isn't easy to do.
He is extremely patient and willing to invest years in his plans to see them pay off, even going so far as establishing a small acting career as a cover identity. He says Sherlock's weakness is that he always wants everything to be clever, so he sometimes misses the more obvious solutions.