Well, almost all our inside jokes come from Arthur and Dazai, they always tease and joke about Mozart calling him a "cat-boy" due to his special liking to to cats.
They also like to joke around with Isaac calling him "apple-kun" due to a... Long story...
Arthur and Theo have various inside jokes between them, normally from stories of the pub and bars they visit.
Comte and I like to count how many times Leonardo do that Italian sign 🤌,we like to bet on how many times he does it.
But we also have an inside joke about Comte being our mother / father, so sometimes when one of us is going outside I like to ask if they have permission from mom.
And people in town once called Leonardo "papi" so we followed the joke, we could say we have a mom and a dad
Isaac and Napoleone like to help kids study for school, so Isaac told them that if they don't go to sleep a monster would break thought their windows and scare them, he meant Dazai being the monster because he likes to enter through the window. We also adapted the joke, so now when someone's going to bed late we tell him that the window monster will visit him... Surprisingly, Jean actually was visited by Dazai by accident. However Jean actually thought Dazai was a monster and attempted to kill him.
We also like to say it will be a lucky day when Dazai uses the main door instead of a window.
I think that would be all the inside jokes... I think we have more but these are already enough to show some of the crazy stories we went thought, my roommates are quiet unique