Grisha Yeager

Dr. Grisha Yeager (グリシャ・イェーガー) is a doctor who lived in Shiganshina District before it fell. He is the father of Eren 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 has 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 try and use hardening abilities, he writes on the ground, "I don't know how to do hardening." He then sloppily writes "My father was by me", referring to Dr. Yeager.