The Ackermann family was once a family of warriors who protected the king, and one of the few bloodlines immune to the Royal family's power. After turning their backs on the monarchy, they were harshly persecuted after rebelling against Karl Fritz and are currently on the verge of extinction. Only two known members are left.
History: At one point, the Ackermanns were the King's sword and one of their vital supporters. After the Eldian Empire collapsed during the Great Titan War a century ago, the family moved inside the Walls that the King Karl Fritz created for the Eldians to live in. However, When the King erased the memories of the people inside the Walls using the power of the Founding titan, the Ackermanns were among those immune to it; the others included the Oriental clan.
Story: During the Battle of Trost District, Mikasa Ackermann recalls the time from her childhood when her mother and father were killed by bandits who were hoping to sell her and her mother to potential buyers in the underground city. She is rescued by Eren Jaeger, killing two of the traffickers before Mikasa kills the third after Eren urges her to fight. While being interrogated by Levi on the whereabouts of Eren Jaeger and Historia Reiss, a member from the Military Police reveals Kenny's surname as "Ackermann," much to the surprise of Mikasa and Levi.
Source: Ackermann family (Anime), available under CC BY-SA 3.0.