The man simply referred to as "Frank" is Dieter Niederburger's nemesis. It is unknown how far back their mutual hatred goes (it is rumored it all stems from Dieter stealing a particularly nice suit of Frank's; an alternate version has Dieter shooting Frank in the head), but the two men have crossed paths more than once over the years and both have emerged alive from each engagement hating each other more than ever.
Frank was Fredo Bessucho's personal bodyguard in The Talented Mr. Fredo. He grew suspicious, and then fearful of Fredo's occult dealings, however, and betrayed his charge, although not completely of his own accord. The angel Uriel, seeking another mortal vessel after its defeat at the hands of Calvin Mayhew, possessed Frank and escaped the battle. Frank/Uriel turned up later in Liberty City and began a killing spree that began with the Bessucho family, nearly claimed Calvin, and ended with the murder of 34 men at the Gardenview Correctional Facility. Frank's body was completely immolated and Uriel banished back to wherever it came from.