Calvin McCullough was a senior officer at SD-6. He headed the psychological warfare and operations division.
After Julian Sark allied himself with SD-6, Sloane explained at a Mission Briefing that Sark had a thorough debriefing with McCullough who determined he could provide credible intel. (Passage, Part 1)
After the End of the SD-6, he spent the last 4 years in Buenos Aires, Argentina, Vaugh and Dixon Came to Question him about his Program, but he knews about being captured and Committs Suicide by Drinking Coffee Spiked with Cyanide.