Gideon

Gideon was one of the four psykers, along with Moore, Wiggum, and Lucy, that was kept behind a forcefield in the Master's Lair beneath the Cathedral. Although he is decidedly insane, no doubt as a result of the Master's experiments with psychic powers in humans, Gideon is still the most comprehensible of the psykers, and the only one who can provide any real information. Gideon informs the Vault Dweller that it is the minds of the Master's failed experiments that inhabit the living walls of the Lair, and that he shares with them a telepathic link. Gideon also informs the Vault Dweller that the only way to suppress the walls' psychic attacks is by wearing a Psychic Nullifier, which he can be conviced to provide. Gideon is still very loyal the Master, albeit in a delusional manner, and speaks very highly of him and his mission in his discussion with the Vault Dweller. He is described as a normal looking human male, fitted with a psychic nullifier. It can be assumed that Gideon died with the destruction of the Cathedral.

According to the Fallout Bible 3 by Chris Avellone, Gideon was a receiving-telepathic-dominant psyker with minor photokinetic abilities, and did not possess the ability to control his telepathy, which meant he required the psychic nullifier to block incoming thoughts. This would explain Gideon's link with the living walls in the Master's Lair.