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 he 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, murdered 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 had a discussion about Eren's wish to join the Survey Corps, Grisha promised him to show him an important secret he was hiding in the basement, and then left to work. An still upset Eren fled from the house and Carla send Mikasa behind him. But while the three were away from home, the Colossal and Armored Titans breached the wall on Shiganshina and a flood of Titans invaded the district. One of the debris that flew out of the wall due to the Colossal Titan's kick fell on the Yeager's house, destroying it and crushing Carla's legs, who was inside. Despite Eren and Mikasa's attempts to rescue her, Hannes had to drag them from there when the Smiling Titan approached. The kids then impotently witnessed as a helpless Carla was murdered and eaten by the Titan.

The Uprising arc
Later on that night, Grisha heard 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, went to the Reiss Chapel and faced the then holder of the Coordinate, Frieda Reiss. The two of them transformed into Titans and fought, but Grisha defeated Frida and ate her, murdering her and stealing the Coordinate. He then proceeded to slaughter the whole family, with the only exceptions of Rod, who barely managed to escape, and Historia, who was living in a farm by then.

Grisha then went to the shelter where Eren and Mikasa were refugees and learned from Eren that Carla had died. Grisha took Eren into the forest and, as he gave him the key to the basement and instructed him to find it by all means, he injected Eren with Titan Serum, triggering a transformation into a Titan. The now mindless Titan Eren grabbed Grisha and bitten him in half, inheriting his Titan Shifting and Coordinate powers. After regaining his human form, Eren found the remains of his father and passed out in shock, where he would be later found by Shadis, who took him back to the shelter.

Eren would later wake up with no memory of that event as a side effect of the Titan Serum, but retaining the key, which he would always wear clinging around his neck. The two kids and their best friend Armin Arlert would apparently leave alone from then on, and two years later they enlisted in the 104th Trainees Squad. In year 850, the three graduated and joined the Survey Corps.