Unlock Bosses/Hidden Characters in Versus Mode.

Zaku II Commander Type (Char Aznable):
On the Mode Select Screen (the one where "Story Mode" "VS Mode" etc., are displayed), press Left, Up, Right, Down, Square, Triangle, O. You will here a sound effect, and Char's Red Zaku will be selectable.

Big Zam:
Finish Story Mode on easy.

Neue Ziel:
Finish Story Mode on normal.

Psycho Gundam Mark III:
Finish Story Mode on hard.

Hydra Gundam:
Finish Story Mode on hard without continues.

RX-78-2 Gundam:
Beat the Story Mode on any setting with Z Gundam, go back and play through Story Mode with any suit but Z Gundam and AcGuy. The Gundam with randomly appear as you play. Defeat it in battle and go on to finish Story Mode and afterward, Gundam will be selectable.

RB-79 Ball:
Beat Story Mode with all 12 starting mode suits on hard.

Get V-Gundam:
beat game on normal with w-gundam on story mode. Then beat game with anyone but acguy or w-gundam

Bosses:
Beat the game in every mode to unlock the 3 last bosses

Thanks to Revolution reader Thomas Conner, Mamadou Mbaye, Andrew Showalter and Jeiarnee Castor!