A/N: An anon on tumblr wanted to see Casey acclimating to his new life away from his dad, so this takes place during/around the "Family Matters" arc.
There wasn't very much Casey had to get used to - not really. The guest room at April's place had been his for about as long as forever, and he kept most of his stuff there to begin with.
The important stuff, anyway.
The necklace his mom gave him before she died hung from the mirror over the dresser, his hockey gear was piled in the closet, the floor was carpeted in extra clothes and comic books, Mr. O'Neil nagged him about keeping it clean. Over the years - after his sister moved out - he stopped going home to his dad's apartment any more than a few scattered nights here and there. When he needed something signed for school, or when his mom's anniversary came around.
He didn't have any warm fuzzies for the old man, but he didn't want to hear about the NYPD fishing his body out of the river on the eight o'clock news, either.
Then his dad hit him, for the first time in almost a year. His dad, a wasted, balding, ex-cop, and Casey laid him out. It wasn't even hard. Wasn't fair, not really. Not with how many empty, scattered bottles there were on the floor. His dad stared at him like he was a stranger, and standing there, Casey felt like one.
That man wasn't family. That apartment wasn't home.
Donnie's eyes had been the brightest thing for miles that night and Casey's skin burned where Donnie touched him - it felt just like when April leaned in to kiss him, intimate and scorching and worth everything in the world. And little Mikey, who Casey loved every bit as fiercely as he loved his little sister, told him in simple, succinct terms, that sometimes family is the people you choose.
"They love you 'cause they like you," the kid had said, earnest, even with those awful bags under his eyes and the new sickly paleness of his skin, "and they want to."
And April was furious with him, because love looked like that sometimes.
And Mr. O'Neil told him to "come home, Casey," like it was so obvious, like it should have been that obvious, and Leo dragged Raph home before anyone else got hurt, and Mikey played tic-tac-toe with him on his practice test, and Donnie reached for his hand under the table, because love looked like that sometimes.