In 1982-83 most of the Hasbro G.I. Joe line was made up of the G.I. Joe special forces members with only one single enemy to fight. The Cobra Commander enemy leader was available by mail-in in 1982 and on the single card in 1983 but until then (for roughly a year or until your mail-in figure arrived) G.I. Joe only had one "enemy" to fight- the generic troop builder figure titled (appropriately enough) Cobra Trooper and the Cobra Trooper Officer (so I guess technically two enemies).
The Cobra Trooper was only a mild modification of the Cobra Trooper with some silver paint additions and a slightly different helmet. The Cobra Trooper is your basic Cobra enemy soldier. In later years, the basic blue cobra trooper would be replaced by the more stylistic Cobra Viper but early on in the comic books and in the toy line this basic trooper was the enemy.
I am probably one of the few who prefers the basic blue Cobra Trooper over the later Cobra Viper. I think the very simple military looking figure with a simple black mask is very "evil" and "terroristic" looking and, to me, more resembles what a renegade, outlaw, paramilitary force would look like than the more futuristic Cobra Viper.