In the "Heatray" level in Multiplayer Deathmatch mode, after the Dark Walker drops, destroy it to get a Damage Amplification.
At the custom character screen, press Triangle, then enter "phayder" as a code to unlock Alanna (Necris).
At the custom character screen, press Triangle, then enter "jihan" as a code to unlock Ariel (Iron Clad).
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.