Brain Damage Gave Me Powers

Most of the time, traumatic head injuries lead to debilitation, disability, or death. But in some special cases, it actually gives the victim a bit of an unexpected upgrade...

Dengar (Star Wars Legend). The bounty hunter Dengar sustained horrific cranial trauma when he had an accident while swoop racing. While saving his life, Imperial surgeons cut out the parts of his brain that normally allowed him to feel most emotions, leaving him only rage, hope (to give him a reason to keep going), and, (accidentally) loneliness. This made Dengar a cold, fearless, and relentless fighter, willing to take logically calculated risks that most right-thinking people wouldn't.

Evan Tanner. The main character of a series of novels written by Lawrence Block, Evan Tanner was a Korean War veteran who had the "sleep center" of his brain destroyed by shrapnel. This gave him a case of "permanent insomnia", which removed his ability to and need for sleep. He used this "extra" time to study up on multiple disciplines, including learning various skills and becoming fluent in several languages.