Hello Lucas!

Right now the maximum level a fighter can reach is 7. They can continue collecting XP once they've reached that level, but they will not be able to level up any further. The collected XP is valid and as soon as a new limit is set, if your fighter has enough XP they will be able to level up further.

I hope that helps! Let me know if you are having issues leveling up before level 7, and I will investigate the issue~