After defeating a character in the park under the Hard difficulty, you'll unlock the un-evolved form of their Bakugan for purchase it at the shop. Complete the game, then challenge the same character again under the Hard difficulty after buying the Bakugan. Defeat that character to unlocked the evolved form of their Bakugan in the shop.
To unlock the following characters Guardian Bakugan at the Bakugan store, win the Ultimate Battle Tournament and defeat Marduk in the last battle. Then, challenge and beat the characters at the park. To unlock the following characters evolved Guardian Bakugan, defeat them twice. Note: You must first purchase the Guardian Bakugan before you can unlock its evolved form.