One Piece: Pirate Warriors 3 contains Treasure Events which are hidden sequences that only play out when certain criteria are met by the player. These range from completing certain actions quickly, to beating levels with specific characters. While none of the Treasure Events in One Piece: Pirate Warriors are especially difficult, some will require a bit of a time commitment since they require specific characters that most players don’t really use often, or are unlocked later in the game.
Characters you will want to focus on leveling up to make your life easier are Zoro, Sanji, Brook, Nami, Ivankov, Buggy, Jimbei, and Boa Hancock. You’ll only need them to be powerful enough for the One Piece: Pirate Warriors 3 Treasure Events that require them, so don’t stress yourself out trying to max out every single one.