Partly as a way to add to difficulty, and as a way to ease balance, I have added certain caps to stats. These caps are as follows:

For characters
HP - 999
AP - 999 (default)
Attack - 500
Guard - 500
Mystic - 500
Speed - 500

For equipment
All stats - 255

As such, without "stat aids," stat-increasing items, or by stacking effects, the highest of any secondary stat (meaning Attack, Guard, etc.) can get is 755. HP can only go above 999 through the use of stat-increasing items or "stat aids." I've yet to calculate the true cap for HP as such, since "stat aids" that increase HP are rare (and their effects are only enacted through level up) and there is a finite number of Life Seeds (the stat-increasing item for HP) in the game.