Board Thread:Questions and Answers/@comment-25100062-20150424161130/@comment-5774380-20150424163000

When one of the kidnappers that murdered Mikasa's parents was strangling Eren, Mikasa felt a kind of "power" awakening inside her, and that gave her the strength to kill the man and save Eren. That's why she's so powerful to this day.

Later, Levi talked to her about that experience and said that he had a moment like that too, and described it as "knowing exactly what needs to be done". That's why he's so powerful too, and said that Kenny Ackerman also experienced that moment.

It seems that's a characteristic of people with the surname Ackerman. Kenny said that the Ackermans are a bloodline of warriors, likely due to that awakaned power.