|Eren! You'll understand the truth someday, if you visit the basement! It will be a harsh, painful journey...but you must reach it! If you want to save Mikasa, Armin, and everyone else...you must learn to control this ability!|
|— Grisha shortly before injecting Eren|
Grisha Jaeger (グリシャ・イェーガー Gurisha Yēgā?) was the father of Eren Jaeger, husband of Carla Jaeger, and the adoptive father of Mikasa Ackermann. He had the power of a Titan and used it to steal the Founding Titan from the Reiss family.
Dr. Jaeger had a gentlemanly appearance. He had straight brown hair and brown eyes with circular glasses. He wore a brown suit with a white cravat and a white shirt with ruffled sleeves. He was also seen wearing a small, black hat with a gray stripe at times.
Grisha's Titan had a body with a notoriously bulkier torso than his arms and legs. He had long, dark hair, elongated ears, small eyes, and his mouth was covered in flesh and a dark, thick beard along with black chest hair. His Titan form had several similarities with Eren's, including their long dark hair, elongated ears, sunken eyes, and a muscular physique.
Although much was unknown about his motives, he appeared on the surface to be a rather calm, calculated man, Grisha's most notable character traits were his hard-headed personality and his intense passion for whatever cause he dedicated himself to.
Grisha was found wandering outside the Walls near the Shiganshina District by Keith Sadies, a soldier of the Scout Regiment. The soldier was very surprised to see him, not only because he was outside the Walls, but also because he claimed to have suffered amnesia and he had no memories about the civilization inside the Walls and the military. Amazed to learn that the people of the Walls were attempting to fight the Titans, Grisha claimed to have amnesia.
Keith brought Grisha back to Shiganshina, where he was imprisoned for illegally entering Titan territory. While imprisoned, he claimed to remember his name. Upon release, Grisha revealed to Keith that he remembered being a doctor, and asked him for knowledge about the world inside the Walls.
Keith took Grisha to a pub in Shiganshina where Keith taught him about the world. Suitably impressed by the life that the people of the Walls had managed to build for themselves, Grisha was surprised to hear that Keith was unhappy with the state of the Walls, and had joined the Scout Regiment in the hope of seeing the larger world. Praising Keith and the scouts, Grisha caught the attention of a nearby barmaid named Carla, who began to chastise Keith for attempting to recruit more people to the Scout Regiment. However, Grisha assured her that even if he was offered the chance, he would not be joining, as the regiment required special people to join to its ranks.
Grisha eventually found work at a hospital. When an epidemic broke out in Shiganshina, Grisha was able to successfully cure those afflicted including Carla and her parents. Grisha and Carla eventually married each other.
He and Carla would later have a son, Eren.
Years later, Grisha was making a house call, with Eren tagging along, to Mikasa's father and mother for an appointed medical examination in the year 844. When he arrived there, he found them dead for hours. He asked Eren if he had seen the girl named Mikasa anywhere, but he was too shocked to answer. He then told Eren to wait for him at the foot of the mountain while he finds the Military Police.
After the MPs and Grisha arrived at the murderers' hideout, Grisha hugged Eren and scolded him for the murders he committed when saving Mikasa, asking if he even understood what he had done. However, instead of continuing to scold him for killing Mikasa's kidnappers, he told him that this time he was lucky and he could have been killed himself. Mikasa, cold and depressed, asked Grisha in what direction her home was and stated that she was cold and wants to leave. In response, Eren gave her his scarf and Grisha told her to come live with them.
Upon learning from Mikasa that Eren wants to join the Scout Regiment, Grisha is more obliging of the idea than Carla, asking his son why he wants to leave the Walls. Upon hearing his son's answer, 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. Before he leaves, he promises to show Eren what he has been hiding in their basement when he returns.
Later, during Wall Maria's fall, Grisha travels to the Reiss family chapel in the night. Bursting in on the royal family, Grisha activates his Titan ability, and attacks the family, intending to steal Frieda Reiss's Titan power. He easily bests Frieda in combat and devours her, before going on to slaughter the entire Reiss family with the exception Rod Reiss, who escapes.
Traveling to Trost District, Grisha locates Keith and enlists his help in searching for his family among the refugees. Upon finding Eren, the two men learn that Carla has been devoured, devastated by the news. Telling Eren that he must avenge his mother, Grisha takes his son into the woods. When Keith tries to stop him, arguing that Grisha should not put such a burden on the boy's shoulders as he might not be gifted like Grisha, Grisha simply explains that the fact that Eren is his son makes him qualified, and orders Keith not to get involved.
In the forest, he entrusts Eren with the key to his basement and injects him with Titan serum, explaining to Eren that their memories will teach him how to use the ability. Transforming into a Pure Titan Eren devours Grisha and returns to his human state, having gained the Founding Titan and Attack Titan.
After Eren finally succeeds in the use of omni-directional mobility gear, head instructor of the 104th Cadet Corps Keith Sadies expresses to himself that Grisha would be proud of his son, Eren, and that he is finally becoming a soldier.
While Eren is chained up in an underground cavern belonging to the Reiss family, Rod Reiss and Historia place their hands on his back, which allows him to experience his father's memories of the night Wall Maria fell.
After Eren recognizes the man Grisha met in Trost to be Keith Sadies, he travels to meet the instructor to find out everything he knows about Grisha.
In the year 844, Dr. Jaeger visits the Ackermanns for a checkup on the mother, who turns out to be expecting. When Eren and Mikasa come across bodies in the forest, Dr. Jaeger makes plans to inform the Military Police.
Grisha was a very skillful doctor, and served many families within the Walls. He was able to cure hundreds of people from a sickness that attacked mankind before the Fall of Wall Maria, earning him respect throughout the Walls.
Grisha was able to turn into a very large Titan; it appeared that he had all of the regular abilities of those with the power of the Titans, like extraordinary regeneration, incredible strength, and endurance when he is transformed. Other than these, it is unknown if he had any other unique abilities. He had some experience with Titan abilities; that was the reason why he managed to kill and eat Frieda Reiss, a Titan that almost matched his size. He stabbed his own hand with a knife in order to trigger the transformation.
- Founding Titan: After eating Frieda, he inherited the Founding Titan and its power. His normally green eyes turned purple in the process.
- Regeneration: As a holder of the power of the Titans, it is believed that he could regenerate his body faster than regular humans, even from fatal wounds.
- Eren Jaeger - Grisha was perhaps closer to Eren than anyone else in his life, surpassing even his relationships with Carla and Mikasa. As a father, Grisha left home for a lot of medical duties; however, he encouraged Eren's dreams to venture outside the Wall and offered to reveal to him the secrets of the basement.
- Carla Jaeger - When Grisha heard of Wall Maria's downfall, he shed tears in fear that his family had perished. Grisha was visibly affected by her death, and when turning Eren to a Titan, is told that her death had made him crazy as his son screamed.
- Mikasa Ackermann - Grisha knew Mikasa when she was a small child from visiting her home as the family doctor. When taking Eren along for a visit, he had hoped that the two would be able to get along as friends. After her rescue from the human traffickers, Grisha showed sympathy towards Mikasa and told her to come live with him and his family. Years later, after the fall of Wall Maria, Grisha mentions her name as he sheds tears in fear that she had been killed by the Titans.
- Mikasa's father and mother - Dr. Jaeger was well known to the Ackermanns as their family physician. They appeared quite friendly when speaking about each other, and Grisha had been known to have been their doctor at least since Mikasa was small, however it appears that as she grew older his visits were less frequent. Grisha seemed quite disturbed at the sight of their deaths.