I would like to leave my input , although its short i would like to leave it all to imagination.
Chaos Knight: Succumbed to chaos and uses it's powers to sow confusion on the enemy.
Blood Knight: A even more twisted version of a knight that enjoy's blood and uses it in battle.
Gunslinger: Now i imagine these guys like some wild west gang, but it doesnt exactly fit their lore that well. They are skilled and fight using dirty tricks, stuns, silences, sleeps. Hence they have all the status debuffs.
Sharpshooter: Again vagabonds, but they are more focused , something like snipers. They also draw power from others; infact they wait for the perfect chance, while their ally's lead the fight (hence they get stat buffs from people in their auras.).
Grand Ranger: Servants of the kingdom, and experts in fighting using bows.
Royal Hunstsman: Something like the equivelent of Sharpshooters but more loyal to the kingdom, since they have all those high military class outfits, and noble postures. Also experts in misguiding and using their teammates to their advantage.
Elementalists: Mystics that have started to live with spirits and use their powers to help their friends, now this might change with the introduction of the summoning mechanic where they might summon elementals.
These are all the classes that i met with and that i know by heart, although i might add more to this, since i really enjoy thinking about this