Strega is a group of three rogue Persona-users who use the Dark Hour for personal benefit. They set up an internet site called "Revenge Request", where users can contract Strega to commit assassinations and other illegal activities. They become enemies of SEES because SEES is trying to put an end to the Dark Hour, as Strega carries out its activities during the Dark Hour.
Near the end of the game, the player discovers that Strega are artificial Persona users created by the Kirijo Group. They were picked up off the street and used as fodder for the corporation's experiments. Because their Personas were not awakened naturally, they must take suppressants supplied by Ikutsuki. The drugs keep their Personas from killing them, but have lethal side-effects.
It is revealed in Persona 3: FES that some of the artificial persona users turned into Shadows.