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.

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.

Additional missions:
Successfully complete the game to unlock another set of missions.

Hint: Kill units easier:
Try to get behind the enemy, then shoot (or hack) at them.