Successfully complete the indicated task to unlock the corresponding character:

Lauren: Always select the second choice for all missions. Kill Lauren in 
Deathmatch in Chapter 2 of Campaign mode. Lauren will eventually become 
unlocked after a subsequent Deathmatch in that chapter.
Scythe: Always select the second choice for all missions. Kill Scythe in 
Deathmatch in Chapter 4 of Campaign mode. Scythe will eventually become 
unlocked after a subsequent Deathmatch in that chapter.
Matrix: Always select the second choice for all missions. Kill Matrix in 
Deathmatch in Chapter 3 of Campaign mode. Matrix will become unlocked after a 
side mission in Chapter 3 in that chapter.
Akasha: Kill Akasha in the final mission.
Loque: Unlocked after a Capture The Flag match in Chapter 5.
Damian: Unlocked in Chapter 4 of Campagin mode.
Kragoth: Unlocked in Chapter 4 of Campagin mode.
Malakai: Unlocked in Chapter 4 of Campagin mode.