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.

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 the wall 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.

When Eren is chained up in a dungeon by Lod Reiss and Kenny Ackerman, Lod 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 Lod Reiss and Historia, who wasn't 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 loudly crying at his father's corpse.