Brain is an unusually large, albino mole rat with an enlarged cranium and the look of an intelligent being. He is the unlikely leader of the renewal cult in the ghoul town of Gecko and seeks to take over the world. He lives in an underground cave adjacent to the Gecko nuclear reactor. He loves cheezy poofs, and will ask to be given some if a visitor has any in their inventory as he can smell them.
In the short term, he seeks to build an alliance between Gecko and Vault City based on power from Gecko being sent to Vault City in exchange for medical research on how to "restore" the ghouls to humanity. The cult is symbolized by the ankh, and its members wear ankh necklaces. The renewal cultists seek to "renew," i.e. become human again.