Win the Ultimate Battle Tournament and defeat Marduk in the final battle. Then, go to the Park and challenge any of the following characters to unlock their Guardian Bakugan at the Bakugan Store. Defeat them a second time to unlock the evolved form of their Guardian Bakugan. Note: You must first purchase the Guardian Bakugan before you can unlock its evolved form.

Dan: Pyrus Dragonoid - Pyrus Delta Dragonoid II
Runo: Haos Tigrerra - Haos Blade Tigrerra
Choji: Aquos Preyas - Aquos Preyas II Angelo/Diablo
Julie: Subterra Gorem - Subterra Hammer Gorem
Shun: Ventus Skyress - Ventus Storm Skyress
Masquerade: Darkus Hydranoid - Darkus Blade Hydranoid
Marduk: Darkus Vladitor - Darkus Battle Axe Vladitor