Not much is known about the mysterious past of Rex but what is, is a Vault City necklace that he wears came from it. He was abducted when he was 20 into the Master’s army where he was exposed to the FEV which mutated him into a monstrous generation 2 super mutant. He loyally served the Master until his death. He wandered California until he found Jacobstown, a super mutant sanctuary made by Marcus. He eventually left and traveled north and met a ranger, a synth, a ghoul girl and an insane doctor.