Agent 347 was a male Human whose ancestors were abducted from Earth 4000 BCE.
Agent 347 was sent to Earth prior 1968, with partner Agent 201, to keep Earth from destroying itself with nuclear weapons.
The two of them used advanced alien technology, hidden in Apartment 12B somewhere in New York City. They hired Roberta Lincoln as a secretary, under the guise of "research for a new encyclopedia".
Both Agent's 347 and 201 were killed in an automobile accident in 1968. When contact was lost, their superiors sent Agent 194, who took over their mission. (TOS: "Assignment: Earth")