There are 6 different fighting styles, all of them relating to a different One Piece character. These styles, while having no immediate use at first, can be leveled up by accepting quests from teacher NPC's (Brawler, Dojo Sensei, Bow Master, Doctor, Weather Wizard, and the Black Leg Trainer).
Brawler- The fighting style that is related to fighting with fists (used by Monkey D. Luffy)
Swordsman- The fighting style that uses swords to defeat opponents (used by Roronoa Zoro)
Sniper- The fighting style that uses guns, bows, and slingshots to defeat opponents (used by Usopp)
Doctor- The "fighting style" that heals allies and mobs, there is no way to harm any mob you come across so this style is best used as a support (used by Tony Tony Chopper)
Art of Weather- The fighting style that uses the Clima Tacts to produce weather balls to combo tempos and deal damage (used by Nami)
Black Leg- The fighting style that uses strong kicks to defeat opponents (used by Sanji)