Numbuh 51 appeared in Operation: E.N.D.. He is a member of Numbuh 86's Decommissioning Squad, a pilot of her S.P.R.A.Y.S.H.I.P., along with Numbuh 91. Numbuh 86 thinks Numbuh 99 is smarter than him. He evidently likes comics, shown when he yells "Comics!" when a piece of newspaper hit his windshield. In Operation: F.U.G.I.T.I.V.E., he and Numbuh 91 crash their ship on a beach while transporting Numbuh 206 to the Moonbase, apparently due to sabotage, and were rescued by Numbuh 86 and Sector V. He also appears in Operation: G.R.A.D.U.A.T.E.S., along with Numbuh 91, flying towards the Delightful Mansion.
Given his covert job, his name is most likely a reference to the US military base Area 51, long rumored to contain paranormal items.