Countershot answers enemy gunmen mid-fight: when one lines up a shot, you fire first. It comes free in Sequence 1 and sits in the Ecosystem tree.
Countershot sits at position 1 of 18 in the Ecosystem tree. Countershot costs no skill points. The twins learn it during Sequence 1. Jacob and Evie each have their own skill tree, so you buy Countershot once for each twin.