Mikasa Ackerman's brother

Mikasa Ackerman's brother 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.

Appearance
Mikasa Ackerman's brother would have appeared to be a healthy baby boy with light hair resembling his father's.

Lost in the Cruel World arc
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, Dr. 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 Dr. Yeager 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.

Trivia

 * In the main storyline, the purpose for Dr. Yeager's visits to the Ackerman family home are never known. Therefore, it is unknown if Mikasa's mother was pregnant with a baby boy in the main storyline.