D. Mccullough is a cut character that was supposed to appear in the F.E.A.R. series. He was to be the head of the F.E.A.R. organization. He has no active participation in F.E.A.R. missions or operations. He most likely acts as an administrative head for dealing with political figures, such as the Senator.
Although being a cut character, his name is displayed in the last Com Link with Norton Mapes in Interval 04 - Infiltration - Watchers. For an unknown reason, his name is displayed as Norton Mapes talks, although it's possible that the developers had D. Mccullough included in a conversation with Mapes.