There are currently 4 major stats for your character. HP, AP, ATK and DEF.
HP is your
After each level, You have the chance to upgrade your Hitpoints / Action points. ATK and DEF can be improved by one's equipment or perks.
Action points are used for special skills or attacks. Basic attacks does not cost AP.
The Defense stat is used to lessen the attacker's damage to you but it will depend on which kind of attack he/she used. 1 def is equivalent to -1 to enemy's damage.