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. It was said that he saved 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.

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.

The Uprising arc
When Eren transforms to a titan to try and use harden abilities, Eren, in his titan form writes, "I don't know what I can do to harden", and then wrote "Father, had me", referring to Dr. Yeager.