Brother is a Dark Cultist that works alone. He wanders from settlement to stronghold, using his Memorization ability to sew discord and destruction. His ultimate goal is to get all the beings in the Great Ruined World to kill one another. While this seems to be happening anyway, it's just taking too long for Brother.
Brother's abilities are boosted by his "potent" descriptor, and of course if he gets into trouble he can always summon a Radiation Golem to run interference while he escapes.
Brother is a great "drop in" villain, either working behind the scenes or directly against the characters.
He could even be used to compel the characters to play out an adventure that is contrary to their natures, or best interests.