Federation Shield for Austin's Zaku 1
After you complete the game once, play practice mission 1 with Austin's Zaku 1. Successfully complete it and you should have a heavy Fed shield like the Gundam's, and a few new supplies.
Char Aznable
Get an "S" rank in Simulation Program 1.
Gaia
Successfully complete the game, then play again. Complete mission 12 a second time to unlock Gai.
Garma Zabi
Get an "S" rank in Simulation Program 2.
Kai, Hayato, and Lt. Agar
Successfully complete the game, then play again. Complete the "Attack Jaburo" a second time. Get an "S" rank on the Federation Simulation mission Burning Sands to unlock Kai and his Guncannon, and Hyato and his Guntank.
LC Garret Schmitzer
Successfully complete all thirteen missions to unlock LC Garret Schmitzer.
Ramba Ral
Successfully complete the game, then play again. Complete mission 8 a second time to unlock Ramba Ral.
Sayla
Get three "C" ranks or below on the Federation missions to unlock Sayla.
All simulations
Successfully complete the game three times. This game will not unlock Program 33. After you unlock it, you can play any character on any sim program.
Additional missions
Successfully complete the game to unlock another set of missions. Successfully mission mode two times on the same file, then check the Simulations. The following missions are now unlocked: Breakthrough II, Midnight Fenrir II, Dobday Rescue II, The Hunter II, Final Defense II, and The Last Fang II.