Strength is a stat in Demon's Souls. In addition to increasing physical defense, Strength contributes to the attack damage of certain weapons, such as hammers, large swords, and polearms. Most weapons have a minimum Strength required in order to wield them effectively; this requirement generally increases along with the item's weight. The player can press the triangle button while wielding most weapons to hold the weapon in both hands, effectively increasing the player character's Strength while wielding it by 50%; e.g. to effectively wield the Bramd in one hand, one needs at least 36 Strength, but a player character with 24 Strength can hold the Bramd with both hands and still wield it effectively, as 1.5(24)=36.
The damage bonus conferred from leveling up Strength decreases drastically once 40 Strength is exceeded.