Strength is one of the six raw abilities, but it is not employed by psy-reliant characters such as your Hero. However, the Hero can increase his strength from the trainer when he aquires a physical attack (such as the knife given to you by Al'al if you choose to work with him.) Increasing your strength still also contribute towards experience for a level up.
Strength is a measure of physical prowess, and determines the base damage of physical attacks. Some characters, such as swordsmen and bowmen, have physical attacks enhanced by psy techniques, in which case the damage of the attack may be a mixture of Strength and Psy Power.
The other raw abilities are: