Yeager family

The Yeager family is the family where the protagonist of Attack on Titan, Eren Yeager, hails from. They also legally adopted Mikasa Ackerman in year 844. They seem to have an origin from outside the Walls, and their basement hides an important secret concerning the Titans. To uncover this secret is one of the main goals of the Survey Corps during the 58th Expedition outside Wall Rose.

History
Around 20 years ago, Grisha Yeager was found wandering outside Wall Maria near Shiganshina District by Keith Shadis, with no other memories besides his name and the fact that he was a doctor. Shadis took him in and got him a job as a doctor in Shiganshina, where he also introduced him to Carla, who worked as a waitress.

Some time later, a plague hit Shiganshina and was cured thanks to Grisha. After her parents and herself were cured, Carla fell in love to Grisha, and they began a relationship. Eventually, they got married and had a son, whom they named Eren.

In year 844, Grisha and Eren went to the Ackerman's house to a medical visit, and found that Mr. and Mrs. Ackerman had been murdered, and her daughter Mikasa had been kidnapped. Grisha went to call the police, and despite his warning to wait downstairs, Eren alone tracked the kidnappers down, killed two of them, and freed Mikasa. After the third kidnapper appeared and began strangling Eren, Mikasa awakened her dormant Ackerman power at Eren's urges and killed the criminal. Mikasa was then adopted by the Yeagers.

Fall of Shiganshina arc
In year 845, after Eren and Carla have a discussion about Eren's wish to join the Survey Corps, Grisha promises him to show him an important secret he is hiding in the basement, and then leaves to work. An still upset Eren flees from the house and Carla sends Mikasa after him. But while the three are away from home, the Colossal and Armored Titans breach the wall on Shiganshina and a flood of Titans invades the district. One of the debris that flies out of the wall due to the Colossal Titan's kick falls on the Yeager's house, destroying it and crushing Carla's legs. Despite Eren and Mikasa's attempts to rescue her, Hannes drags them from there when the Smiling Titan approaches. The kids then impotently witness as a helpless Carla is murdered and eaten by the Titan.

Later on that night, Grisha hears the news of the fall of Shiganshina and, enraged to the Reiss family due to their refusal to free humanity from the Titans despite possessing the power to do it, goes to the Reiss Chapel and faces the then holder of the Coordinate, Frieda Reiss. The two of them transform into Titans and fight, but Grisha defeats Frieda and eats her, murdering her and stealing the Coordinate. He then proceeds to slaughter the whole family, with the only exceptions of Rod, who barely manages to escape, and Historia, who is living in a farm by then.

Grisha then goes to the shelter where Eren and Mikasa are refugees and learns from Eren that Carla has died. Grisha takes Eren into the forest and, as he gives him the key to the basement and instructs him to find it by all means, he injects Eren with Titan Serum, triggering a transformation into a Titan. The now mindless Titan Eren grabs Grisha and bites him in half, inheriting his Titan Shifting and Coordinate powers. After regaining his human form, Eren finds the remains of his father next to him and passes out in shock, where he is later found by Shadis, who takes him back to the shelter.

Eren later wakes up with no memory of that event as a side effect of the Titan Serum, but retaining the key, which he always wears clinging around his neck from then on. The two kids and their best friend Armin Arlert apparently live alone for the next two years, after which they enlist in the 104th Trainees Squad. In year 850, the three graduate and join the Survey Corps.