Karim's parents had gotten married and had his older brother and him, and for awhile, they were a happy family. But, one day, Karim's father— who was actually from outside the walls— was messing around with some certain experiments, and he left some things out.

Karim had been told to never get into his dad's things, but his curiosity got the better of the two year old, and he found some weird stuff in a syringe. He picked it up, but when he heard footsteps, he quickly put it back and turned around, but the test tube fell and broke on the bottom part of the back of his neck, on his spine, and he accidentally injected himself with a Titan serum.

Karim, having become a Titan, ate his brother— then he went unconscious. When he woke, his father told him he was a monster. Apparently, he'd eaten not only his brother, but he'd crushed his mother.

They left that specific location and moved to a different, far location. Karim was treated like a beast by his father, never like a human child. He was even locked into a dog cage whenever his father left him home alone. It lowered the kid's self esteem and confidence, and he lost any feeling of self worth. He truly believed that he was a monster and that he was a bad kid.

Because of his quick healing abilities, he never told anyone of the abuse his father put him through. How could he? There was no evidence. If anything, they'd only find out his secret. So he never spoke a word about it.

His father led him outside of the walls and kept him blindfolded before leading him to Paradis, where he put Karim under hard, difficult, and strict training. He was more abusive than the actual instructors.

Karim was one of the titans there when the Armored and Colossal breached Wall Maria. He was there with them, and he was so fast that he was able to create a miniature tornado, and able to bring destruction to some of the houses just by running through them. Why did he do it? Because he was three years old, he was told to do it, and he wanted to make his father proud.

One day, in the year 850, he accidentally transformed into his Titan, and he ended up eating his father while in a mindless state since he hadn't gotten full control over it quite yet. When he came to, in his human form, his memories were fuzzy, but he could remember eating his father. It horrified him.

Being alone and not knowing what to do, he wandered back to the place his mom had been killed at within the walls— he ran quickly enough to jump really high/climb the wall and managed to get in. It caused a commotion in the process for those in Wall Rose. He found policemen there, and they found out who he was (minus the fact he's a Titan shifter) before informing his uncle, Eld Jinn, of what had happened to his sister and that they'd found his orphaned nephew.

     A note about his past and his father: his father, being from outside the walls, was a Marleyan. So once Karim had become a Titan shifter, he began teaching him the false teachings of Marley, telling him that Titan shifters were evil and must be killed, and that all within the walls were demon offspring that must also be killed.

     Therefore, Karim is to be part of "The Mission", and Annie, Reiner, and Bertholdt have been searching for him. Karim, despite his father being abusive, wants to do what would make him proud since it's a feeling he'd longed for, and his father said he'd be proud if he achieved their goal.

     The poor kid has been brainwashed by his dad and those outside the walls, so he believes them to be bad, but at the same time, after he meets the Survey Corps, he has trouble believing that they truly are. He becomes torn. But, only after a day of being with his aunt and uncle, certain events unfold and he couldn't keep his secret any longer. He told them about the fact he was a Titan shifter, and he told them about Marley and about the fact that Titans were once humans.