WARNING: The following contains spoilers for multiple chapters of Attack on Titan, by Hajime Isayama, Dezi Sienty and Alex Ko Ransom, available now in English from Kodansha.
Over the course of Attack on Titan, Mikasa and Levi Ackerman have shown impressive strength and proved themselves able to do battle with Titans in ways entirely unlike other humans. Thanks to an "awakening power," which gives them physical strength far beyond humans' normal capabilities, the Ackermans are able to channel the fighting power and experience of their ancestors, essentially utilizing the strength of Titans without becoming one themselves. However, the origin of their strength remained shrouded in mystery... until recently.
The Ackerman Family's Origin
We first learned more about the Ackerman family's unique powers through the stories of Kenny and his nephew, Levi. Kenny was a mentor to young Levi, whose mother died of an illness when he was very young. Kenny is legendary, considered by some to be "Kenny the Ripper" because of his severe treatment of his victims. Levi is one of the most powerful of the soldiers in Attack on Titan and probably the most powerful Ackerman. Upon learning more about his family, he confirms that his powers were "awakened" and asks his cousin, Mikasa Ackerman if her own powers were awakened in a moment of intense stress. She answers that they were, long ago -- thinking back to her traumatic childhood.
We learn the precise moment that this change happened in Mikasa in Attack on Titan Chapter 112, "Ignorance," when Eren Yeager reveals to Mikasa the truth of her lineage. The Ackerman family was designed by the Eldian Empire, which combined the Subjects of Ymir and Titans to create a race that could serve and protect the Eldian kings. A member of the Ackerman family is activated when a figure acting as a host commands them to act. Mikasa's powers were activated by Eren when he told her to attack the robbers who assaulted them when she was just a child. Eren even goes so far as to speculate Mikasa's powers bind her to him, as he serves as her host.
What being a host means, however, is unclear. Eren has the power of three Titans, two of which he inherited from his father. He isn't said to have royal blood, so Mikasa serving him as a host seems questionable, which their childhood friend, Armin points out. Eren isn't exactly a trustworthy character, either, and it's entirely possible he's hiding something he learned in his father's basement from Mikasa. Eren also beat Armin relentlessly after Armin disagreed with Eren's analysis of Mikasa's powers. But there's no doubting the power of the Ackerman family, host binding or no host binding.
The Ackerman Family's Top Killer Moments
The Ackerman family were made killers through the experimentation done on them by the Eldian Empire long ago. Kenny, as signified by his grim moniker, was known for his brutality in his role as a member of the Military Police Brigade's First Interior Squad. Levi is considered the strongest soldier of humanity, serving in the Special Operations Squad of the Survey Corps. Mikasa is also considered a top soldier in her role as an officer in the Survey Corps.
Kenny's top moment comes in his first appearance in the first episode of Season 3 when he makes a flamboyant reappearance back into Levi's life. Only a figure known as "Kenny the Ripper" would be so daring in his attack on humanity's top soldier. Kenny's knowledge of Levi's fighting style, since he was the one who taught him it, also gives him a unique advantage. But Levi can beat his uncle at his own game, proving that he's only grown in his powers.
Levi's top moment arguably comes when he brings down the Beast Titan singlehandedly -- taking him by surprise and very nearly killing him. Levi has never let anyone or anything stand in his way, making him one of the series's most powerful warriors.
Mikasa is also considered one of humanity's best. She's a genius tactician and soldier, and not a bad leader, either, as we learn from her powerful speech about her strength when compared to her fellow soldiers, in "Small Blade: The Struggle for Trost, Part 3." Mikasa is defiant to the often grim fates that await the characters of Attack on Titan. It isn't just her physical strength that awes, but her emotional strength, as well.
The Ackerman family's secret was long withheld from readers, viewers and Ackerman descendants alike. The secrets that have since come forth are game-changing, but in a way, they prove what viewers already know: The Ackermans are the toughest of warriors, and they'll fight to the bitter end.