Unreal Tournament 3 - Unlockables Characters

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.

Play as Alanna Necris
Press Triangle at the custom character screen, then enter phayder as a code.

Play as Ariel Iron Clad
Press Triangle at the custom character screen, then enter jihan as a code.

Easy Damage Amplification:
Play the "Heatray" level in Deathmatch in multi-player mode. After the Dark Walker drops, destroy it. You will get a Damage Amplification every time.