Spread out across the 63 day campaign you will receive five characters as post mission events. They are completely random. However If you get a character you don't want/need, you can dashboard out before completing the event, and relaunch the game. Upon relaunch you will have to manually end your previous mission. At that point you can possibly be offered an alternate character. Rinse and repeat as you see fit. However sometimes the games seems focused on the same 1-3 characters.
This achievement is for recruiting Marian Janiak, Mechanic.