Play as Alanna (Necris) - At the custom character screen, press Y, then enter "phayder" as a code to unlock Alanna (Necris).

Play as Ariel (Iron Clad) - At the custom character screen, press Y, then enter "jihan" as a code to unlock Ariel (Iron Clad).

Bonus characters - 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.

Easy Damage Amplification - In the "Heatray" level in Multiplayer Deathmatch mode, after the Dark Walker drops, destroy it to get a Damage Amplification.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Lock and Load (10 points): Complete Chapter 1.
Soldier's blood (30 points): Complete Chapter 2.
Does not compute (30 points): Complete Chapter 3.
Not in Kansas any more (30 points): Complete Chapter 4.
I'm not on a holy war (60 points): Defeat Akasha and complete the campaign.
Just business (30 points): Complete Chapter 2 in Insane.
Bag of bolts (30 points): Complete Chapter 3 in Insane.
Open war (30 points): Complete Chapter 4 in Insane.
Fear the Reaper (60 points): Defeat Akasha and complete the campaign in Insane.
I need some backup (10 points): Complete 1 mission in co-op.
Got your back (20 points): Complete 10 missions in co-op.
Thanks to all the little people (40 points): Complete a campaign in co-op.
I see how it is (10 points): Complete an Instant Action match in every game mode.
Untouchable (20 points): Reach 20 kills against a god-like bot in instant action without dying.
Like the back of my hand (40 points): Collect every power up on every map.
Spice of life (20 points): Play a match with every mutator, using only 1 per match.
Brain Surgeon (10 points): Get the "Head Hunter" award in 10 matches.
Don't taze me bro! (10 points): Get the "Combo King" award in 10 matches.
Goo God (10 points): Get the "Biohazard" award in 10 matches.
Pistolero (10 points): Get the "Gun Slinger" award in 10 matches.
Shard-o-matic (10 points): Get the "Blue Streak" award in 10 matches.
Hammerhead (10 points): Get the "JackHammer" award in 10 matches.
Strongest Link (10 points): Get the "Shaft Master" award in 10 matches.
Have a nice day! (20 points): Get the "Rocket Scientist" award in 10 matches and "Flak Master" award in 10 matches.
Big Game Hunter (10 points): Get the "Big Game Hunter" award in 10 matches.
Armadillo (10 points): Get the "Roadkill" award in 10 matches
Jack of all trades (10 points): Kill an enemy with every vehicle (except hoverboard).
Ace (10 points): Get the "Top Gun" award in 20 matches.
Deathwish (10 points): Get "Bullseye" (self-destruct to destroy an enemy) award in 20 matches.
Seeing Red (10 points): Have over 20 minutes of "Berserk Time".
Never Saw it Coming (10 points): Have over 20 minutes of "Invisibility Time".
Survival of the fittest (10 points): Have over 10 minutes of "Invulnerable Time".
Delivering the Hurt (10 points): Have over 20 minutes of "UDamage Time".
Hat trick (20 points): Achieve a Hat Trick in 10 CTF or VCTF matches.
Being a Hero (10 points): Return 100 Orbs in Warfare matches.
Flag waver (20 points): Completed 100 CTF matches played to at least 3 captures.
Thirty minutes or less (20 points): Complete 100 VCTF matches played to at least 3 captures.
Paint the town red (20 points): Complete 100 Deathmatch or Duel matches played to at least 20 kills.
Connect the dots (20 points): Complete 100 Warfare matches played to at least 3 points.
Serial killer (20 points): Get the "Killing Spree" award in 20 matches.
Sir Slays a Lot (20 points): Get the "Monster Kill" award in 20 matches.
Killjoy (10 points): End sprees in 20 matches.
Off to a good start (10 points): Get the "First Blood" award in 40 matches.
Lets get it on (20 points): Complete a multiplayer match in every game type.
Around the World (20 points): Win a multiplayer match on every map.
Get a life… (50 points): Get 200 kills in multiplayer on 50 different days.
Just warming up (10 points): Finish playing a "Versus" Ranked match.
Winning is the only thing (10 points): Finish with the highest points in a Ranked match.
Equal opportunity destroyer (20 points): Win a Ranked match in every game type.
Ranked Champion (50 points): Win 500 Ranked matches.