Luck is the seventh stat in this world. It is neither a mental nor a physical trait. It is a little more unpredictable in character creation. Everyone besides a Fateless gets a base luck of 3d6. By spending 2 ability points into Luck you instead get 4d6, drop the lowest. For 5 points, you get 3d6+6 (Drop the Lowest). Finally, for 10 points, you get 10+2d4.
Luck represents how important you and your goals are in the Fate of the world; people with high luck are destined for greatness, and will find their goals almost supernaturally achieved. People with lower Luck will fail at many of their goals; they stumble over things they didn’t notice, attacks tend to find their way to the chinks in their armor, and floor boards in old houses break beneath their feet, sending them crashing below.
Luck has several effects behind the scenes, both the luck of an individual, and the luck of a group as a whole. These are complicated and intricate calculations, like the thread of fate itself. However, those with higher luck are more likely to find items that are suitable to their needs, narrowly avoid trouble, find good events, and have stylish victories.
However, there are two ways in which your Luck modifier affects you the player in noticeable ways;
Your hero points per level is increased by your luck modifier (But not decreased by it. You get a minimum of 1 every level). A character can have a total number of hero points equal to 2+(Luck Modifier*2), Min 2.
You also add your Luck modifier to Critical Confirmation attacks, and your AC when determining if a critical hit confirms against you.