Head: Duke (modified)
Torso: Gung-Ho
Upper arms: Grunt
Lower arms: Tunnel Rat (I think)
Waist: Snake Eyes
Legs: Coils

As the filecard states, Mercer was once a Viper Commander. After leaving his team stranded, Mercer decided that being a Viper Commander wasn't cracked up to be. So he decided to leave his Viper team behind and join the G.I. Joe team.

Only one condition, he had to serve under Sgt. Slaughter's men...Taurus and Red Dog. Together, they would be known as Sgt. Slaughter's Renegades. Whenever a mission was occurring, he would have to participate.

In the end, Mercer made a wise choice and went with the good guys.