When you get enough experience points to level up a dialog box pops up and you recover your HP and MP. Then you get to choose which stats to level up on your character. The Stamina and Will stat will each raise one point each level, and whichever stat you choose gets two points added to it. In the Japanese version of the game, instead of choosing a stat, the player is presented with a set of job types, or classes, instead.
Power / 戦士 (Soldier)
Increases the Power rating the most. Raises Attack Power to your advantage in battling monsters.
Stamina / モンク (Monk)
Increases the Stamina rating the most. Maximum HP and Defense rating rise, so you take less damage in battles.
Wisdom / 魔道士 (Sorceror)
Increases the Wisdom rating the most. Raises maximum MP, allowing you to use more magic.
Will / 賢者 (Sage)
Increases the Will rating the most. Increases the speed of the gauge, allowing you to MAX attack more frequently.
In order to calculate your HP and MP, and the experience needed for each level-up, the following formulas are used by the game:
HP = ((Stamina x Stamina) / 10) + 19 rounded down
MP = Wisdom x 0.94 + 5 rounded down
Exp = (L^3 x 1.03) + (L^2 x 3) + (L x 12) rounded down, where L is your current level.
This means that if you choose Stamina every time you level up, you can max out your HP to 999 by the time you reach level 50, or if you choose Wisdom every time you’ll max out your MP to 99 by level 50.