Aptitudes are a number starting at 0 that determine a character's proficiency with the weapon. Should be balanced so that it never really gets past 100.
0 is average, totally normal, no bonuses, so Aptitudes only improve a character's skill with a weapon.
Fist, Dagger, Sword, Axe, Bow, Wand - These all function the same way. Bonus Damage = Aptitude / 8. Bonus Accuracy = Aptitude / 6. Connecting with a weapon gives you 0.01% to 0.15% aptitude with that weapon type.
Evasion - Bonus EVADE = Aptitude / 8. Increases by 0.01% to 0.15% each time an attack misses you.
Shields - Bonus BLOCK = Aptitude / 8. Increases by 0.01% to 0.15% each time you block an attack.
Throwing - Bonus ACCURACY for throwing = Aptitude / 6. Increases by 0.01% to 0.2% each time you connect with a thrown item.