Stephen Shellen plays Section operative Marco O'Brien. Formerly a wily and tenacious homicide detective, O'Brien was recruited into Section four years ago by Nikita after accidentally stumbling onto her true identity. Although O'Brien has transformed into an impressive and shrewd anti-terrorist operative, he still harbors resentment against Nikita for taking his old world of crime and punishment away from him -- even though she did so to protect his life. O'Brien eventually warms up to Nikita, and although he knows he can never replace Michael, he nevertheless becomes a trusted and loyal partner.