Grisha Yeager is the father of Eren and foster father of Mikasa. 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.
[Spoiler] In the basement of the Yeager family house, he hides a secret which he refers to as the "truth." Upon meeting Eren after the fall of the Wall Maria in the year 845, he injects him with a series of chemicals and amnesiacs, entrusting him with the key to the "truth," before disappearing. [/Spoiler]
(Source: Wikipedia)
Note: Grisha Yeager is the official English translation by Kodansha.
