Head: Shipwreck (Spy Troops) modified
Torso, crotch and legs: Viper (VvV)
Upper arms: Leatherneck (VvV) and Grunt (Spy Troops)
Lower arms: Grunt (Spy Troops) modded with Gung-Ho (JvC) hands

Holster: Chap Mei modded with scraps from a Roadblock (Spy Troops) arm
Pistol: Chap Mei
Rifle: Marauder
Backpack: Alley Viper (Spy Troops Army Builder)

My Joeverse holds Mercer to his essential origins as a former Cobra Viper who has defected. But I've fleshed out the details some.

Mercer left Cobra because he realized he'd joined them for the wrong reasons. But he still believed in a lot of what Cobra stood for and many of the people he left behind there were friends.

Because his defection involved a Joe (to be explained in a future dio) and ended up with him essentially federal custody, he was facing treason charges.

Due to the unusual circumstances, Hawk was able to pull some strings and have Mercer pressed into service in exchange for his eventual pardon on all counts of crimes against the government. To earn his freedom, he simply has to betray his former friends and everything he thought he believed in by helping shut Cobra down.

Readers of my "Beginnings" dio will note the character's uncanny resemblance to a certain founding Joe team member. There's a good reason for that.