Mr. Keller is the head of the Keller family, which took refuge in the locked bunker of the National Guard depot.
He appears as a glowing one in their hideout, having mutated due to numerous trips outside of the bunker with faulty, or damaged, radiation protection.
This is supported by the Diary of Candace Keller, which notes that he frequently left the safety of the bunker. The G.E.C.K. ID of the glowing one in the bunker - "MisterKellerGlowingOne" - supports this as well.
He is the only member of the Keller family that is still "alive", so to speak.
Keller Senior appears only in Fallout 3.