Stefano was a foreign hit man who worked for Franco Bertinelli, the Huntresses father. he once got tired of taken orders and set out to kill Franco and his wife and become the leader of the gang. He did, the trauma caused Franco's daughter Helena to become the huntress.
Stefano has super human strength. In addition to this he has powerful resistance, enough to withstand Black Canary's sonic blast which is revealed to be able to kill other meta humans however he walked away from it unharmed.
In the show Justice League Unlimited Stefano appears in the episode "Double Date" going by the name Steven. He has the same origin as in the comics however it is revealed that a rival kidnapped his son Edgar and testified against him. In return the feds put him in the witness protection program. However he and his gang break out in order to see his son. The Huntress and Question ambush him ad she is about to kill him. However when she sees him standing with his son she remembers the night her parents were murdered and spares him.