You get to create attributes for characters. These are typically fixed but can change via script commands. Later there will be derived numbers which figure out what the stats actually become when level, equipment, and such are factored in. That is the part that makes the player stronger as they level up.

Or you could rely much more on the attributes. You can also use them to assign numbers to players as additional currencies (Job Points, etc.).

Invisible to the player only means it won't show up the system menu when viewing a player's stats. This can also be used to hide it there and display it elsewhere using your own method.