i finished one battle royal (which can be short ) and spent hours and hours and hours on story mode (30+ hours) and didnt finish it. Yes story mode can be played in multiplayer and thats whats really fun about the game !

you have your character level which can reach 99 and your job level which can reach 7 for each class.
Their are two benefits from rising you class level : 1- unlock new attacks
2- unlock new jobs (classes)

the job system works in a smiler way to final fantasy tactics, you start with three classes in the beginning of the game : warrior, mage, rogue.

reaching level 7 with any of the classes will unlock "cleric". but to unlock more powerful classes you would need to reach max job level with two classes.
so here is a mini chart of how things work:

mage + thief : alchemist
mage + warrior : spell sword
thief + warrior : ninja
cleric (alone) : monk

there is i think about 24 classes in total ? the game is huge and has a massive amount of content. No one knows about it cause it isnt a ps3 exclusive or contains mario characters.