Complete the game 100% (collect all cards). You can now choose to have restricted cards to be no longer restricted.

Blue Millennium Booster: Defeat Rare Hunter ten times.
Blue/Green Millennium Booster: Defeat Strings ten times.
Dark Ruler Hades Booster: Defeat Bandit Keith ten times.
Gold Millennium Booster: Defeat Odion ten times.
Guardian Sphinx Booster: Defeat Pegasus ten times.
Jinzo Booster: Defeat Weevil ten times.
Purple Millennium Booster: Defeat Umbra and Lumis ten times.
Red Millennium Booster: Defeat Arkana ten times.
Relinquished Booster: Defeat Rex Raptor ten times.
The Masked Beast Booster: Defeat Maku Tsunami ten times.
Thousand-Eyes Restrict Booster: Defeat Bonz ten times.
Toon Summoned Skull Booster: Defeat Espa Roba ten times.
Yamata Dragon Booster: Defeat Joey Wheeler ten times.
Zombrya the Dark Booster: Defeat Mai Valentine ten times.