Nephestis is a Sheftu spy for the Tlak'kahn. It's his shape-shifting abilities that gives him an advantage. He framed Major Gus Bonner by impersonating him on PR6-308. He tarnished Major Bonner's reputation even more back at Stargate Command by using his stolen image to stun the Stargate technician and Cadet Harrison who was in the control room at the time. He used the technician's DNA to open the iris and activate the Stargate to let the Tlak'kahn into Stargate Command. (INF: "The Decision")
When Gus Bonner and his team returned to Earth through the Yucatan Stargate, Nephestis took the appearance of General Stoneman to trick the team into disarming their lasers and tried to separate the team. In order to leave Gus and Draga alone, so that they would be easier to capture with Da'kyll. (INF: "The Long Haul")