I've had this in drafts for like. I don't even KNOW how long
Raph is definitely a biter. Sometimes, he bites his brothers as a way of showing affection without trying to be obvious. He doesn't know that they know.
Raph often has sensory overloads, mainly from noise. Usually he runs and hides in the sewers somewhere to calm down, but if something is happening and he's forced to be in the lair, his brothers go quiet to let Raph calm down. Leo apologizes the most if he triggers a sensory overload(once Raph calms down).
Donnie absolutely HATES the feeling of felt.
Leo often thinks of if, in the future, they would be loved for saving the world, despite being mutants. He is a pessimist as well, so you can tell how he thinks of it.
Mikey often scares human teens into dropping spray cans so he can make art for his brothers and Splinter. Casey is the only person who knows, and usually helps him from boredom.
Mikey often tries to get Donnie and Leo to get some pets. Raph got him a snake as a pet but it got eaten by one of the mutant squirrels.
Leo apologizes a lot for minor things.
Donnie has OCD.
Casey loves anime. (This is technically canon but nobody has talked about it so)
April has that 'millennial humor' as her humor.
Donnie has accidentally met nearly every single other variation of themselves. His favorites were Rise!Donnie(Dontron) and 2003 Raph(Fisty McFistyson).
All of them flinch way more Tham expected.
Donnie loves pickels. He gets so happy when food places mess up his order and put pickles, he will happily take any pickles the others don't want.
Mikeys most favorite pizza ever was that one he made himself.
Raph really likes BBQ chicken pizza.
Leo actually prefers spaghetti of all kinds but also loves pizza and any topping/kind.
For clarification, I do not care if any of these are contradictory to canon. I'm the captain around here.