bridge agent an operative who acts as a courier or go-between from a case officer to a secret agent in a hostile area
case officer an operative who also serves as an official staffer of an intelligence service
codetalker,
windtalker a secret agent who was one of the Navajos who devised and used a code based on their native language; the code was unbroken by the Japanese during World War II
foot travel by walking; "he followed on foot"; "the swiftest of foot"
noc an undercover agent who is given no official cover
undercover agent,
spy (military) a secret agent hired by a state to obtain information about its enemies or by a business to obtain industrial secrets from competitors