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.