Unlock Choco:
Beat the 30 round arena trial in Rueloon using Darc's Party
Unlock Diekbeck:
Beat the 20 round arena trial in Cathena using Karg's Party
Begin Game with Karg:
After finishing the game one time you will have a new option called "New Game+" that lets you start with Karg with King Statuette, 3 Diamond coatings, 999 Spririt Stones, 10 Bountiful Fruit, 50 Dilzweld Bombs, and 100,000 Gold.
