Yukio had been living with his brother for fifteen years before Rin was let in on The Secret. Yukio, born knowing said secret, knew all there was to know about his brother (including the stuff Rin himself didn't know).
But now, uncountable empty milk cartons later, Yukio was beginning to realize that maybe the whole "demonic powers of Satan" thing would bring out traits in Rin that had never seemed very important before.
Like Rin's new-found love of milk. (Which was draining the extra spending cash they had considerably.)
Or his persistence to laze around in the sunlight (although lazing around was in no way a new habit).
His reflexes, while always decent, had skyrocketed to the point where he wa so light on his feet that he was like a ghost-when he wasn't running his mouth.
(Not a literal ghost, of course. He was a demon. Not a ghost.)
It had dawned upon Yukio why this behavior seemed so familiar when he walked in on his brother lounging on his bed, Kuro wrapped snugly on his chest.
Rin reminded him of a god damn cat.
(A very dangerous, flame-wielding cat, but nonetheless.)
When Yukio offhandedly mentioned this to his older twin (in the hopes of embarrassing him enough to quell the cat-like persona) Rin had licked a sharpened canine and meowed at him.
It was never brought up again.