Ackerman clan

The  is one of several families that live within the Walls. They were traditionally a bloodline of warriors close to the crown, but after refusing to follow the ideology of the First King, they suffered intense persecution from the monarchy that left them on the brink of extinction and currently there are only few living members left.

History
The Ackermans were one of the families that lived in the world prior to the appearance of the Titans. They were acknowledged as a bloodline of warriors close to the crown and acted as the monarchy's right hand entrusted with humanity's survival; probably thanks to the "awakened power" characteristic of them (see Awakened power). Some members of this family survived the predation by the Titans that started more than 100 years ago and moved within the Walls that the First King built to save the remnants of mankind.

After the First King erased the memories of the people inside, the Ackermans were one of the few bloodlines that showed immunity to this mental manipulation, the others being the Asian races and the noble families. However, along with the Asian bloodline, they refused to follow the King's ideology of a world ruled by Titans as the true peace and gave up their status in order to turn their backs on the monarchy. Fearing his inability to control them, the King plotted a persecution against them, bringing both races at the brink of extinction.

At the end, the head of the Ackerman family offered his own life to ensure the survival of the bloodline and was executed. The rest of the generation avoided to tell their children about humanity's lost memories in order to protect them from the monarchy's purge. As a result, the following generations grew up without that knowledge.

Nevertheless, not even this stopped the oppression on them, and several generations later the Ackermans were still pursued, with some of them seeking refuge in the Underground City and others fleeing to the mountains at the edge of the Wall.

The human hunting only stopped after Kenny Ackerman met the then King Uri Reiss; with the latter apologizing for the actions of the prior kings, stopping the persecution, and accepting Kenny as his right-hand man.

Awakened power
Along with their immunity to mental manipulation, a special ability displayed by some members of the Ackerman family is a kind of "awakened power" not seen in any other bloodline so far. This ability is described by Levi Ackerman as "knowing exactly what needs to be done". Users of this power exhibit physical abilities far above the average soldier, such as great strength, speed, agility, accuracy and coordination.

In consequence, these users are acknowledged as extremely powerful warriors, with Levi being known as "Humanity's Strongest Soldier", Mikasa Ackerman as the best soldier of her generation and "as valuable as a hundred average soldiers", and Kenny Ackerman being considered by Levi as even more dangerous than himself and capable of killing more than 100 members of the Military Police without being caught.

This power doesn't seem to be present in all members (or at least not active), but arises when the person experiences certain moments in their life that awakens it, and remains active for the rest of the user's life.

The only Ackerman seen experiencing this moment so far is Mikasa, who was normal until the age of 9 when three human traffickers killed her parents and kidnapped her. After witnessing one of the kidnappers strangling Eren and being urged by him to fight, Mikasa awakened her dormant power and was able to kill the criminal with a single stab from the back that pierced his heart, displaying a level of speed, strength and accuracy abnormal for a girl of her age; with the floorboards even snapping under her foot as proof of the immense strength she unleashed.

Levi stated that himself and Kenny also had similar moments that awakened their powers, though the exact circumstances are unknown.

Mikasa also experienced a drastic change in her personality after that event, going from being a somewhat cheerful girl to a much more serious and focused person very similar to Levi. However, it is unknown if this change in personality is related to the awakened power or to the trauma of losing her family.

Trivia

 * The Ackermans are said to be traditionally a bloodline of warriors who acted as the right-hand men of the monarchs; and curiously enough, the three known "awakened" Ackermans have always ended up acting as the loyal right-hand men and bodyguards of someone: Mikasa follows Eren everywhere and is constantly trying to protect him; Levi follows Erwin and he is the only person Levi blindly trusts and obeys; and Kenny was for many years the bodyguard, right-hand man and close friend of Uri, whom he kept respecting even years after Uri's death.