Lyle Rennhull is an officer in US Air Force Intelligence and a KGB informant.
As a member of Air Force Intelligence, Renhull was able to provide classified documents on the American's Strategic Defense Initiative anti-ballistic missile program.
In May 1981, he was recruited by Sanford Prince without the permission of Elizabeth. Elizabeth is initially skeptical of Rennhull, partly due to compulsive-gambler Sanford Prince demanding $50,000 for the information. Rennhull does, however, come good on the deal at a meet with a disguised Philip, providing documents on technology so advanced it would be 50 years before being remotely operational. ("The Colonel")
Rennhull later alerts the FBI to Sanford's attempts to recruit him. After Sanford's removal from the FBI building, he goes to Rennhull's house, ranting that his betrayal had ruined his life. Rennhull kills Sanford, allegedly self-defense by shooting him twice in the head. ("Comrades")
FIn October 1987, he's a brigadier general and Elizabeth attempts to blackmail Rennhull into handing over a sensor for the Soviet Dead-Hand system. When their meeting date arrives, Rennhull refuses to cooperate. Elizabeth gives him another delivery date and Rennhull shows up armed. A dramatic scene ensues with Elizabeth attacking Rennhull, attempting to wrestle the weapon away from him. Landing atop of him, Elizabeth maneuvers the gun under his chin and squeezes his hand to pull the trigger; his brains splatter all over her head. Hearing the shot, Paige leaves her post and comes running, horrified to see her mother covered in blood and brain matter. Elizabeth later lies to Paige and to Philip that he committed suicide, something Philip does not believe, and openly questions in front of Paige. ("Tchaikovsky")