|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 had also been seen wearing a small, black hat with a gray stripe.
Mysterious Titan form
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.
Years before the fall of Wall Maria, Grisha Jaeger 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.
That night, after Wall Maria's breach, Grisha reunites with his son. In tears and on the verge of hysteria, Grisha gives Eren the basement key and forces him to take an injection that causes him to lose his memories. As he is injecting Eren, he tells Eren that he must get to their basement, and that Eren must learn to control his power in order to save Mikasa's, Armin's, and everyone else's lives. The injection causes Eren to transform into a Titan, and Grisha is devoured by his own son.
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.
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.