The Ackerman Family is an Eldian family living within the Walls. Traditionally, they were a bloodline of warriors that protected Eldia's King, but were persecuted to the brink of extinction after refusing to follow Karl Fritz's ideology. The head of the Ackerman family is styled as a Marquess or Marchioness.
The Ackermans lived in Eldia prior to the raising of the Walls. They were the accidental result of the Eldian Empire's experiments with the Subjects of Ymir and Titan science. They were designed to protect Eldia's King, and acted as the monarchy's right hand, entrusted with the survival of their people. After the Eldian Empire collapsed during the Great Titan War over a century ago, the family moved inside the Walls that King Fritz created for the Eldians to live in.
When the King erased the memories of the people inside using the power of the Founding Titan, the Ackermans were among those immune to it; the others included the Asian Clan and those who would later become the noble families inside the Walls. However, along with the Asian clan, they rejected the King's idea of peace, and turned their backs on the monarchy. Fearing his inability to control them, the King persecuted them, bringing both races to the brink of extinction. At the end, the head of the Ackerman clan offered his own life to ensure the survival of the bloodline and was executed. The rest of the generation avoided telling their children about the world before the walls 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: Several generations later, the Ackermans were still being pursued and some of them sought refuge in the Underground City. Others fled to the mountains at the edge of the wall, but someone got in the way of their business which made them stay poor. The purge only truly stopped after Uri Reiss, the king at the time, met Kenny Ackerman in an assassination attempt. The former apologized for the actions of prior monarchs on the latter's family, stopped the persecution, and accepted Kenny as his right-hand man.
The Ackermans have an "awakened power" displayed by some members of the clan. Levi Ackerman describes it as "knowing exactly what needs to be done," and it lets the Ackermans exhibit physical abilities far above the average human. Eren Jaeger learns from Zeke that this is because they were the result of the old Eldian Empire's fiddling with the Subjects of Ymir. They can manifest the Power of the Titans as humans, without becoming a Titan or inheriting any of the Titan Shifters. When awakened, the Ackerman gains the battle experience from every single Ackerman before them via "paths."
This power is not automatically active, however, and has to be triggered. The conditions are to find a "host" (a person to protect), and when facing death, to obey an order from the host. This will activate the instincts found in the Ackerman's blood to protect the host, as the Ackermans were engineered to protect the king. For example, Mikasa was a normal girl until the age of 9 when three robbers killed her parents and kidnapped her. Eren Jaeger came to her rescue, and when they were facing death, he told her to "Fight." This awakened Mikasa's dormant power, and she was able to kill one of the criminals with a single stab and snap the floorboards with the strength of her foot. Levi has stated that he and Kenny also had similar moments that awakened their powers.
At times, an awakened Ackerman will experience sudden headaches. This is the result of the Ackerman's "true self" resisting the Ackerman instinct of protecting the host