Mikasa Ackerman's brother (ミカサ・アッカーマンの弟 Mikasa Akkāman no otōto?) would have been a baby born into the Ackerman family in the year 845. He appeared in the visual novel Lost in the Cruel World as well as the Attack on Titan: Lost Girls novel and manga of the same name.
During the battle for Trost District, Mikasa Ackerman experiences a vision of a reality where her mother and father were not killed by human traffickers. In this reality, Grisha Yeager visits the Ackerman family with his son Eren in order to check up on Mikasa's mother, who is pregnant with what she expects will be a baby boy. Upon hearing of Mrs. Ackerman's pregnancy, he hopes that the boy will become a soldier when he is grown up.
After a short time, Mrs. Ackerman begins to grow ill, increasing the frequency of Dr. Yeager's visits. Her condition, which is discovered to be gestational hypertension, continues to worsen until the point where she was advised to move to Shiganshina District, where Grisha could provide more immediate care in case of an emergency. Some time later, Mrs. Ackerman successfully gives birth to her son in Shiganshina District with her husband and the Yeager family by her side.
However, the unfortunate truth of the matter is that Mrs. Ackerman dies at the hands of a trio of human traffickers in the year 844. In reality, Mikasa Ackerman's brother dies in the womb long before he is born.
- In the main storyline, the purposes for Grisha Yeager's visits to the Ackerman family home are never known. Therefore, it is unknown if Mrs. Ackerman was pregnant with a baby boy in the main storyline.