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 and husband of Kalura Yeager. Hannes said he was the one 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 - something his son would inherit - and he also had 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.

After the fall of Wall Maria, a tearful Dr. Yeager injects a serum into Eren's left arm, which Eren believes is responsible for giving him the ability to shift into a Titan. Dr. Yeager tells Eren that the serum will cause memory loss and gives him the key to the basement, saying that all of the answers can be found there.

Dr. Yeager then disappeared, and had not been seen for five years since then.

104th Trainees Squad arc
After extensive training, Keith Shadis expresses to himself that Dr. Yeager would be proud of his son, Eren, and that he's finally becoming a soldier.

The Uprising arc
When Eren transforms into a Titan to make some experiments, he's asked by Hange to try and communicate with them. He states by writing on the ground that he doesn't know how to harden his body. He then sloppily writes "My father... to me", referring to Dr. Yeager. As he's resting after the experiment ends, Eren wonders where his father is and why he thought of him during the experiment.

When Eren is chained up in a dungeon by Rod Reiss and Kenny Ackerman, Rod and Historia place their hands on Eren's back and allow him to relive his father's memories. They depict Grisha moving toward the Reiss chapel and finding his way underground during the night Wall Maria was breached. Once there, he seemingly transformed into his Titan form and killed every member of the Reiss family except Rod Reiss and Historia, the latter not there at the time. During this attack, he battled at least one other Titan Shifter. He then destroyed the chapel and gave Eren the key to their basement after finding him. He then injects Eren with a serum that causes him to transform into a Titan. This Titan Eren eats Grisha and gains the abilities of a Titan Shifter, before reverting to human form and loudly crying at the grisly remains of his father.

It is later revealed by Rod that Grisha, in his Titan form, entered the Reiss family chapel, where they were gathered. In the chapel, he appeared with a frightened expression as he confronted them, with Frieda protecting them. He used a knife to stab his hand and subsequently transformed, and Frieda as well by biting her hand. Rod reveals that Grisha was after Frieda's Titan power, and that Frieda's Titan was had an "invincible power", but she lost due to lack of experience. Grisha ate Frieda, stealing her Titan powers, and brutally slaughtered the remaining Reiss family. Among the deceased were Rod's fourteen-year-old son Dirk and twelve-year-old son Abel, both of whom he crushed with his hand. He also stomped on ten-year-old Florian, who died with his mother, and squeezed to death Rod's eldest son Ulklin. He destroyed the chapel and Rod was the only one who managed to escape.

Trivia

 * The appearance of Grisha's Titan can make a reference to the giant Surt. According to Norse mythology, 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.