Grisha Yeager was an Eldian doctor who originated from the Liberio internment zone in Marley, and was one of the Eldian Restorationists. He was sent on a mission from "the Owl" to infiltrate the Walls and take the Founding Titan from the Reiss family, and was given the power of the Attack Titan in order to do so. Grisha was taken inside the Walls by Keith Shadis, and settled in the Shiganshina District. He documented the details about Eldia, Marley, the Titans, and his past in three books that he hid in his basement.
In Marley, he was married to Dina Fritz and had a son named Zeke. After being sent to Paradis Island, he re-married to Carla Yeager and had a second son, Eren Yeager.
Grisha was a fairly tall, slender man with straight black hair neatly parted down the middle, a thin beard and mustache, and gray eyes. He wore a gray suit with a black vest and a white shirt, a plain bolo tie, and round glasses. On the left side of his chest, he had a large cross scar that he carved into himself as an oath and rite of passage to join the Eldian Restorationists.
Appearance: Grisha was a fairly tall, slender man with straight black hair neatly parted down the middle, a thin beard and mustache, and gray eyes. He wore a gray suit with a black vest and a white shirt, a plain bolo tie, and round glasses.
Personality: Although he appeared on the surface to be a rather calm, calculated man, Grisha's most notable characteristics were his hard-headed personality and his intense passion for whatever cause he dedicated himself to. Grisha's hard-headed tendency was most evident during his childhood, and usually to his own detriment.
History: Grisha was born in Liberio in the year 806 on the continent of Marley. One day, in the year 817, he and his sister ran out of the internment zone chasing after a blimp and were found out by two Public Security Authorities officers.
Grisha Yeager in the story: Upon learning from Mikasa that Eren wants to join the Survey Corps, Grisha is more obliging of the idea than Carla and asks his son why he wants to leave the Walls. Eren's answer, that the deaths of the many soldiers who have already served in the Survey Corps will be a waste if no one continues the fight, galvanizes Grisha to resume his mission to retake the Founding Titan from the Reiss family. Grisha announces that he will be departing, telling a bewildered Carla that there is no point in trying to stop an inquisitive mind like Eren's.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.