WARSAW

When you first play the game and do the tutorial, you will receive three of the characters. The rest you will encounter throughout multiple playthroughs. Spread out across a 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.

Cumulatively across your playthroughs, you will need to acquire each character in the game. There are 17 in total. While some have their own individual achievements tied with recruiting them, several do not.

For this achievement you will need to collect the following:

(Starting three)
Jadwiga Mazuek, Paramedic
Krzysztof Zielinski, Home Army Soldier
Kazimierz Lis, Scoundrel

(Everyone else)
Bronislaw Wajner, Berling's Soldier
Jan Cieslak, Watchman
Wladyslaw Skupien, Doctor
Ernst Budnik, Executioner
Marian Janiak, Mechanic
Henryk Toporek, Officer
Janos Wigner, Liberated Prisoner
Helena Kwiecinska, Telephone Operator
Wanda Mularczyk, Sapper
Anna Brzeska, Sharpshooter
Karol Fedorwicz, Silent Unseen
Martin Luscher, Turncoat
Franciszek Wronski, Scout
Konstanty Wolowicz, Uhlan