When the following tasks have been completed during gameplay on the correct difficulty setting the corresponding extras becomes unlocked.

Unlock Sgt. Frank Delgado:
Pop 50 heads.

Unlock Fr. Paul Rawlings:
Pop 100 heads.

Unlock Exlposive Cultist:
Disintegrate 50 enemies.

Unlock Flamethrower:
Disintegrate 100 enemies.

Unlock Cpt. Devin Ross:
Using melee attacks kill 50 enemies.

Unlock Cultist:
Using melee attacks Kill 100 enemies.

Unlock Lt. Abigail Black:
Using Ghost Bullet kill 50 enemies.

Unlock Grenadier:
Using Ghost Bullet kill 100 enemies.

Unlock Inanna:
Complete 'Skin' on Hard difficulty setting.

Unlock Ki:
Complete 'Flesh' on Hard difficulty setting.

Unlock Governor Cassus Vicus:
Complete 'Rome' on Easy or Normal difficulty setting.

Unlock Bishop Malthus St. Claire:
Complete 'Crusades' on Easy or Normal difficulty setting.

Unlock Psychic Commander:
Complete 'World War 2' on Easy or Normal difficulty setting.

Unlock Child Crusader:
Complete 'Crusades' on Easy or Normal difficulty setting.

Unlock Arnold Leech:
Complete 'Al Khali' on Easy or Normal difficulty setting.

Unlock Pyxis Prima:
Complete 'Sumeria' on Easy or Normal difficulty setting.

Unlock Sir Richard de Gray:
Complete 'Out of the Frying Pan' on Hard difficulty setting.

Unlock Sgt. Wilhelmina 'Billie' Church:
Complete 'Al Khali' on Hard difficulty setting.

Unlock Brother William of Auxere:
Complete 'Motley Crew' on Hard difficulty setting.

Unlock Crossbowman Crusader:
Complete 'Tortured Souls' on Hard difficulty setting.

Unlock Agent Muriel Green:
Complete 'The Den of Souls' on Hard difficulty setting.

Unlock Deceased Devin Ross:
Complete 'Still With You' on Hard difficulty setting.

Unlock Corpses Behemoth:
Complete 'River of Blood' on Hard difficulty setting.

Unlock Legionary:
Complete 'The Low Road' on Hard difficulty setting.

Unlock Gladiator:
Complete 'Temple of Pain' on Hard difficulty setting.

Unlock Enlil:
Complete 'Spiritual Guide' on Hard difficulty setting.

Unlock Antadurunnu:
Complete 'The God Seal on Hard difficulty setting.

Unlock Operation: Godseal:
Complete 'Exorcism' on Hard difficulty setting.

Unlock Cpt. Xavier Jones:
Complete 'Man Down' on Hard difficulty setting.

Unlock Cpl. Simone Cole:
Complete 'The Tomb' on Hard difficulty setting.

Unlock Warrior Crusader:
Complete 'Sewers' on Hard difficulty setting.

Unlock Nanna:
Complete 'Blood' on Hard difficulty setting.

Unlock Ninlil:
Complete 'Skin' on Hard difficulty setting without being incapacitated.

Unlock Utu:
Complete 'Blood' on Hard difficulty setting without being incapacitated.

Unlock Sumerian Puppet:
Complete 'Flesh' on Hard difficulty setting without being incapacitated.

Unlock Centurion Textius Longinus:
Complete 'Decadence' on Hard difficulty setting without being incapacitated.

Unlock Blind Behemoth:
Complete 'Morituri te salutant' on Hard difficulty setting without being incapacitated.

Unlock The Jericho Team:
Complete 'Rivers of Blood' on Hard difficulty setting without being incapacitated.