. He doesn't really speak unless he's spoken to, and if he does speak without being spoken to, he appears to be anxious the moment he realizes it.
. He's been a Titan shifter since he was two years old.
. His brother was supposed to be the Titan shifter (an ally to Marley at that, rather than to the Eldians), but he accidentally became a Titan and then ate his brother, gaining the shifting ability.
. He has asthma attacks when he gets too stressed or anxious.
. He has a slight fear of soldiers and tall men. So if a tall man is a soldier, he's especially afraid. If you speak kindly to him though and prove that you are kind, he will become less afraid.
. He has a leather necklace with a sharp shark's tooth on it.
. He absolutely loves his aunt, and he loves Victoria Bowman/Ackerman, who he sees as an older sister. (Victoria belongs to SpamChan.)
. He feels safe with Reiner and Bertholdt because they were always kind to him even when his father and others were not.
. He feels safe with Annie as well, since she was often with him when he was with Reiner and Bertholdt.
. Sometimes, his dad would beat him just because he was angry, not because he did anything, and tell him that he was a monster.
. Karim is afraid of brooms and mops. At first he is hesitant to say why, but the reason is because his dad would hit him with the broom and mop. He liked to use the handles an awful lot.
. Since he had quick healing and wouldn't die, his dad would get angry enough to stab him with a knife whenever he was angry enough. This was how Karim accidentally became a Titan and ate his dad in blind rage— though he wasn't aware of it until he'd woken up from a blackout.
. Karim hates baths and will refuse to take one. He has to be forced to bathe because of his refusal to. It's later found out that he's afraid of baths because his dad would hold his head under the water during baths until he would pass out.