You can unlock three secret, overpowered soldiers by changing the name of one of your soldiers. Go to the soldier’s customization, and then to their character information page, and alter their name according to the following guidelines.
(Note: while using one of the secret characters, your soldier’s previous attributes are erased and you can’t earn any achievements/trophies.)
Sid Meier (a Psi Operative, named after the game developer) – Change the soldier’s first name to Sid and last name to Meier.
Peter Van Doorn (an Assault Build Ranger Colonel, he is meant to be the popular NPC from XCOM: Enemy Unknown) – Change the soldier’s first name to Peter and last name to Van Doorn.
Beaglerush (a Grenadier, named after a well-known streamer of XCOM games) – Change the soldier’s nickname to Beaglerush. It doesn’t matter what the first name and last name are set to.