Unstoppable I gives you more breathing room between hits before your combo counter drops. It is a one-point Combat pick available after Sequence 1.
Unstoppable I sits at position 8 of 18 in the Combat tree. Unlocking Unstoppable I takes 1 skill point. You need to complete Sequence 1 before you can buy it. Jacob and Evie each have their own skill tree, so you buy Unstoppable I once for each twin.