Unlock William Carter from The Bureau: Xcom Declassified:
Select a hero unit at your barracks and then go to the Customize option and change his name to William Carter (William in the first name area and Carter in the last name area). William Carter is an Assault Class pre-equipped with Titan Armor, Alloy Cannon, and Plasma Pistol. You will not be able to earn Achievements.
Unlock Special Units:
Enter one of the following names during squad customization to unlock the corresponding special unit. You will not be able to earn Trophies.