Grant Ellis was a government agent and part of a project to develop the mind control drug Analytic Histamine.
Ellis worked on the project and decided to enter a fake relationship with Susanne Modeski when she threatened to expose the project. He pretended to help her when she was operated on. Ellis let her believe that he would work against the project and run away with her. In 1999, they were supposed to carry out their plan at DefCON. Instead, he let Timothy Landau take the drug and use it to assassinate Modeski.
He started the conference with a poker game in which The Lone Gunmen had infiltrated. Ellis caught on to John Fitzgerald Byers not being who he said he was, and Melvin Frohike not really being a server.
Ellis thought he saw his plan work out during the conference, as Richard Langly seemed to have killed Modeski during the conference. He was surprised to find out she was alive in his hotel room. She confronted him, but they were interrupted by Landau who promptly killed him. (TXF: "Three of a Kind")