He is portrayed by Charles Halford.
Jemm was one of the most ruthless convicts ever sent to Fort Razz in the Phantom Zone. Eventually, he and all other prisoners escaped when Fort Razz crashed onto earth. At one point he was captured by the D.E.O. and imprisoned at their base. To countereffect his psychic abilities, the D.E.O. devices special headband inhibitors and imprisoned Jemm in a special glass cage which removed his powers.
At one point, a huge earthquake stuck National City. The temporary power outage allows Jemm to flee his cage. The D.E.O. seals the base, trapping themselves inside their base with Jemm. While director Henshaw and two agents try to find Jemm, the psycic ambushes them with his powers and kills the two agents. Henshaw manages to return to the control room where the rest of the D.E.O. agents are bunkered up. He tells them that both inhibitors were destroyed during Jemm's attack. As Jemm used his psychic abilities to read the mind of one of the deceased agents, he knows everything there is to know about the D.E.O. base, including how to open the cells of the other kryptonian convicts.
Not trusting Henshaw, agents Danvers and Donovan head out themselves to recapture Jemm. While walking past the corpses of the two agents, they find that despite Henshaw's assurances the inhibitors are still working. While trying to take one of it, Donovan is psychically attacked by Jemm who orders him to attack Danvers. However, Donovan struggles and tries fighting Jemm. Not knowing how long he can fight off Jemm's influence, Donovan tells Danvers to run.
Danvers goes on alone, eventually meeting Henshaw. Both are contacted by Donovan who wants to regroup with them, but Henshaw is not fooled and answers that he knows that Jemm is speaking. Jemm then answers that they only delayed the inevitable and that he will free all prisoners and break through the shield around the base. After Jemm stops the contact, Danvers forces Henshaw to chain himself to the wall because she doesn't trust him anymore. After he has done so, Danvers contacts Jemm and tells him that he doesn't have to free the prisoners as she has the access code for the shield and will be waiting for him in the control room. Once Jemm arrives, Danvers shoots him into the chest with a machine gun as well as a shotgun, blasting the crystal off his head. Jemm then overpowers Danvers, but before he can kill her he is killed by Henshaw who attacks him from behind and breaks his neck.