The Ultimate Inventory System does not Handle character stats out of the box.
There are simply too many things related to character stats that are out of scope for an Inventory system (character skills, damage, gameplay features, etc... ) that we decided not to implement it within UIS, instead we give a simple example that you can build from.
The "Player Character" script is specific to the demo scene. You can have a look at the code, copy/paste it in your own project and start modifiying it. At least this will give you a good starting point.
We also explain in the documentation some different ideas on how to implement stats:
In many games Items can be equipped and/or used to affect the character stats and/or the weapon the character is carrying. Using the attribute system . . .