Vasily Zeitsev was a male, high-ranking operative in Section 31. He preferred dimmed lighting and had a heightened perceptual acuity allowing him to monitor many surveillance screens which each rotated at an updated pace.

In 2379, he headed a Section 31 operation from the secret sub-subbasement under the Federation Embassy on Qo'noS. Klingon Imperial Intelligence had been gathering intelligence on him for quite some time when their agent Lorgh told Ambassador Worf about their existence.

Believing Zeitsev to be the 'real' head of Starfleet Intelligence on Qo'noS, Worf went to the sub-subbasement and confronted Zeitsev when he needed to know how he could get access to the Klingon Fleet Command Codes. Zeitsev told him of the possible locations to get access and provided him with a host of top-of-the-line and prototype espionage devices to aid him in his quest, including a scanner/signal jammer, dermal voice pouch, neural disruptor, and holomask (based on mobile emitter technology).

After getting the codes, Worf hid the devices in his office in the embassy. When he later returned, both the tools and everything in the sub-subbasement was gone.

Zeitsev had fled to an undisclosed location to meet with L'Haan and Dietz, two other Section 31 operatives to discuss what to do about the current Tezwan situation. (TNG novel: A Time to Kill)

Along with his fellow operatives, Zeitsev continued to monitor the actions of Azernal and Quafina, including their hiring of the merchant freighter Caedera. Once Zife's illegal actions were known to a select group of Starfleet admirals who forced him to resign, he, along with L'Haan and Dietz, was responsible for escorting Zife to give his resignation, and then executing Zife, Azernal, and Quafina. (TNG novel: A Time to Heal)

Zeitsev was probably named for famous WWII Russian sniper Vasily Zaytsev. The surname literally means "son of rabbit," but is nevertheless regarded as perfectly reasonable, and is a fairly common name in Russian-speaking regions.