The Secret Service Agent is a minor character who appears in "Serum to Codfish Cove".
An agent of the secret service, who is after the spies, visits Huey, Dewey and Louie. He explains the situation to them and they offer to recover the stolen blueprints and take the serum to its destination. The agent agrees and deputizes them as secret agents.
Later, the kids find their tired uncle sleeping in the snow outside Codfish Cove and the spy ready to attack him. They knock the spy out, replace the blueprints with the serum and wake Donald with a gunshot. Donald manages to arrive to the hospital.