"We are the men of intrinsic value, who can strike our fortunes out of ourselves, whose worth is independent of accidents in life, or revolutions in government: we have heads to get money, and hearts to spend it."
During missions, a mercenary squad is trained at a merc compound. They have no build time, and arrive nearly instantly at the compound by drop pod. There is a short cooldown until the mercenary squad may be trained again. The squad size varies by the unit, and there is a limit to the number of squads that may be trained for each mercenary per mission. Mercenaries benefit from armory upgrades and laboratory research that benefit their base unit.
Mercenaries have unique unit models—60% of the model is altered to ensure the models are unique, but still recognizable.
Mercenary unit models are recycled throughout the campaign for other units, usually when that unit is meant to be a stronger version of the base unit. For instance, the Jackson's Revenge model is reused as the Loki, and the War Pigs model is reused for the Umojan Marine. In the final mission of Heart of the Swarm, members of Mengsk's Elite Guard use mercenary models.
On higher level difficulties, some pre-placed enemy units will be replaced by their mercenary counterparts.