David Sinclair is an FBI Agent in Don Eppes' unit, and the only other one to appear in all six seasons of Numb3rs.

David is an intelligent, dedicated and compassionate law enforcement official, often arguing for the average citizen when other agents get rather gung-ho. He is loyal and hard-working, and as of "Cause and Effect" headed off to be Special Agent in Charge of a group of his own, just like his mentor. David had a difficult childhood in many ways, not least due to a lack of money, and he tends to be the quickest in the unit to believe a hard-luck story, although there are exceptions, especially when his generally reliable instincts tell him there's something wrong.

David, like Don, seems married to the job - not that he doesn't have girlfriends, but they rarely last any length of time.

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