If you prefer not to use cheats, you can still unlock all classes by focusing on specific evolution paths. Classes are unlocked by reaching Level 15 in the prerequisite class.
For most players, the safest approach is using the European version's debug menu on PPSSPP with a backed-up save file — this gives you the maximum unlock potential with the most control over what you activate. The debug menu's DEBUG ALL ON/OFF option can unlock everything at once, but consider using the more granular options to unlock only what you need and avoid breaking the game.
Level up Piekron to Level 8 and Yarida to Level 10.