Father of Eren Yaeger and caretaker of Mikasa Ackerman. He is a renowned doctor who was well respected in the District of Shiganshina after he saved the village from a mysterious viral epidemic by developing vaccines. He also provides a medical house call service to the people of Shiganshina and to various towns in addition to having a network of contacts within the Corps of the Shiganshina District. He is married to Carla Yaeger.

In the basement of the Yeager family house, he hides a secret which he refers to as the "truth". Upon finding Eren after the fall of Wall Maria, he injects him with a series of chemicals and amnesiacs and entrusts him with the key to the "truth", before mysteriously disappearing. This strange serum resulted in Eren gaining the ability to transform into a titan.