Gunslinger I keeps your revolver in the fight by reloading it on the fly during combos. It is a level 3 skill costing two points.
Gunslinger I sits at position 9 of 18 in the Ecosystem tree. Unlocking Gunslinger I takes 2 skill points. You need level 3 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Gunslinger I once for each twin.