How? Did you edit the game files somehow?
And no, characters stay in their own campaign so you can't grind the same guy 50 times. It seems like there will be some areas wher eenemies respawn, though.
Out of curiosity, I used a software to adjust numbers in the PC demo.
As I was curious about those weapons and armors outlook (weapon shining effect/armor appearance), I adjusted the gold to buy all the things from the 1st town blacksmith.
And I adjusted the xp to see what skills I will have for Lucas once hitting the max Lv30. Then I discovered that I only got 9 visible skills for Lucas. And I found out that after I met Anjali, she will be also Lv30 that means the level of companions depends on your main character's level.