Col. Frank Simmons worked for the NID, but had his own agenda. He was very much opposed to the members of SG-1, and was introduced in the episode, 5.03 "Ascension".
Col. Frank Simmons became the NID representative with whom the SGC interacted after Col. Harry Maybourne was arrested for treason. Simmons claimed to have the best interests of the SGC at heart, but considered all the members of SG-1 inadequate to their task. On the mission where he captured and imprisoned a Goa'uld which had taken over billionaire Adrian Conrad, Simmons shot Jack O'Neill in the back. Later, Conrad's symbiote transferred to him and he was killed in a fight with O'Neill onboard the Prometheus.