Grisha Yeager

Dr. Grisha Yeager (グリシャ・イェーガー) was a doctor who lived in Shiganshina District before it fell. He was the father of Eren Yeager, adoptive father of Mikasa Ackerman, and husband of Kalura Yeager. Hannes claimed that Grisha was responsible for saving hundreds of lives from a plague that hit mankind. He was also a Titan Shifter.

Appearance
Dr. Yeager had a gentlemanly appearance, his work clothes consisting of a brown suit and slacks, and fashioning a cravat. He was also bespectacled, always wearing round glasses. Dr. Yeager had brown hair parted neatly down the middle - which his son would inherit - and a light beard and mustache.

Fall of Shiganshina arc
At the beginning of the story, Dr. Yeager is living in Shiganshina District with his wife, Kalura; his son, Eren; and his adoptive daughter, Mikasa Ackerman. He keeps the basement of his house locked and doesn't allow anyone inside. Before Wall Maria fell, he promised Eren that he'd show him what was in the basement when he got back from a journey.

After the fall of Wall Maria, a tearful Grisha injects a serum into Eren's left arm, which Eren believes is responsible for giving him the ability to shift into a Titan. Telling Eren that the serum will cause him memory loss, Grisha gives him the key to the basement, saying that all of the answers can be found there, and that "their memories will guide you." Grisha then disappeared and was not seen for the next five years.

104th Trainees Squad arc
After Eren finally succeeds in the use of Three Dimensional Maneuver Gear, Trainee Legion commander Keith Shadis thinks to himself, "Today, Grisha, your son has become a soldier!"

The Uprising arc
When Eren transforms into a Titan during experiments conducted by Hange, she asks him to try and communicate with her and the other Survey Corps personnel. He writes on the ground that he doesn't know how to harden his body. He then sloppily writes, "My father... to me," referring to Grisha. As he rests after the experiment, Eren wonders where his father is and why he thought of him at that moment.

While Eren is chained up in an underground cavern belonging to the Reiss family, Rod Reiss and Historia place their hands on his back, which allows him to experience his father's memories. During the evacuation of the Wall Maria territory, we see Keith Shadis a few steps ahead of Grisha and looking back at him. Then we see Grisha moving toward the chapel that then stood aboveground over the cavern, finding his way underground, and pleading with the gathered members of the Reiss family: Rod, his wife, their four sons, and Frieda. He then transformed into his Titan form and killed every Reiss except Rod, who escaped, and Historia, who was not present. During this attack, he battled at least one other Titan, a female, whom he ate. He then destroyed the chapel. Later, in the woods, he gave 10-year-old Eren the key to their basement in Shiganshina. He then injected Eren with a serum that caused him to transform into a Titan, although not his later Rogue Titan form. The transformed Eren consumed Grisha, reverted back to his human form, and began to wail in horror at the grisly remains of his father.

Rod later reveals that Grisha confronted the Reiss family in the chapel with a frightened expression on his face. Frieda stood in front of her family, protecting them. Grisha then stabbed himself through the hand with a knife and transformed; Frieda shifted by biting her hand. Rod states that Grisha was after Frieda's Titan power, and though it was "invincible," she was not experienced enough to defend herself. Grisha ate her, obtaining that power, and then brutally slaughtered the remaining Reiss family. Among the victims were 14-year-old Dirk and 12-year-old Abel, both of whom Grisha crushed with his hand. He also stomped on 10-year-old Florian and the boys' mother, and squeezed Rod's eldest son Ulklin to death. Finally, he destroyed the chapel. Rod was the only one who managed to escape.

Trivia

 * The appearance of Grisha's Titan may be a reference to the giant of Norse mythology. According to mythology, he is the leader of the fire giants in the south, the ruler of Muspelheim, and at Ragnarok his hordes will rush to the north like a southern wind to overwhelm the gods.