In the mid-2370s, AdmiralBullock was the role played by a member of Species 8472 in the training simulation on the Terrasphere 8, designed to accurately simulate Earth. "Bullock" was a high-ranking member of Species 8472 who reported directly to "Boothby."
Later, after Species 8472 confirmed that they'd been discovered, "Bullock" reported to Boothby that Voyager was approaching, and charged the biosphere's weapons. When the two tried searching for a diplomatic answer, he was sent, along with Boothby and Valerie Archer. His stance was firmly that the Federation couldn't be trusted, even after Boothby had been convinced. After a truce was reached, he had his assistant, Kinis, give Neelix a tour, while he personally showed Tuvok around before Voyager left. (VOY: "In the Flesh")
It is unclear if there was a real Starfleet Admiral Bullock or if the persona was created by Species 8472. According to Boothby and Kathryn Janeway, the simulation was incredibly accurate of Starfleet Headquarters and its personnel.