I have this modern AOT HC that Jean had a guinea pig as a kid and without telling his mom (who had earlier said no) went out and bought his a friend and well, they were opposite sex so.. you can imagine Mrs Kirsteins horror when she comes to clean his room one day and he's been hiding guinea pig babies!
He cries when she makes him give them all away and his friends all take one. Everyone takes good care of them, except Eren who accidently forgot it existed and well 💀.
Carla apologizes to Mrs Kirstein and they decide not to tell Jean, but he tells Connie who spills the beans and Jean is pissed and to this day, that's why Jean still holds a grudge against Eren.