Headcanon (canonverse) where Eren adopts a stray cat and keeps it with him in secret, hiding it in his room. It’s chubby, well-groomed and excellent at catching mice, and he names it Levi. Only Armin and Mikasa know about the cat, because it’s against the rules to keep any animal - until it vanishes one day, a few months later.
The trio search everywhere but return empty-handed, much to Eren’s dejection. He was generally terrible with animals but had a soft spot for his grumpy cat (which had a tendency to wake him up by sitting on his chest and staring at him with those creepy yellow eyes - but hey, at least its fur was soft.)
When he’s sure everyone is asleep, Eren sneaks out of his room and resumes his search. After a couple of hours, he stands in front of the last empty room in the entire castle - but its door is jammed. “Fuck..” he murmurs. “Levi! Are you in there?” hearing no response, he sighs. “I’m sorry I ever thought you were a creepy prick, okay? Please come back to me… I miss you!” he said in sleepy desperation. “Oh, and I won’t ignore you again. I’ll shower you with cuddles and attention whenever you want it, little demon. Just come back!”
He touches the doorknob again and it swings open. “What did you say?” Captain Levi Ackerman (who, incidentally, was also the person Eren has been madly in love with for the past 3 years) sputtered.