Diabolik is a ruthless master thief. He typically steals from criminals (and has no issue with killing them if need be, but rarely, if ever, kills the innocent or the police), and has a set of lifelike masks which he uses to fool his opponents, assuming every identity at his will. He seems to have a deep knowledge in many scientific fields, including chemistry, mechanics and computers.
In his first appearances, Diabolik was a more straightforward villain who did not hesitate to murder anyone in order to accomplish his deeds. He was later given a more “Robin Hood”-like persona and was shown stealing essentially from criminals, in order to soften the series’ violence and amorality.