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.
(Source: Wikipedia, edited)
Note: Grisha Yeager is the official English translation by Kodansha.
