This achievement is for the campaign only!
There are 8 different playable characters in the game (4 male & 4 female)
Play every Level/Chapter of the 3episodes as a different character and you will unlock this.
You need to play from start (lobby) to finish (lobby) with the same character to count.
If you start with the first episode and the first chapter, and always choose a different character for each chapter, you will get this before the end of episode 2.
The characters are not different by abilites or so, they just look different.