It is a supernatural trainer class. In the core book. In the manga and anime there was a character named riley who was a lucario trainer but he had the ability to see auras and eventually he learned to actually attack with it and taught his lucario how to use it as well. Ash in two episodes has the ability to use aura as well.
The character Idea I have is actually a child who ended up lost in the woods, would have died except that A lucario saved its life teaching him how to survive until one day a trainer found them and convinced him to follow him to town.