Cas felt a little like the wind was knocked out of him at the idea of Dean from being from so long ago. That was before Hell. Before he'd pulled him out and they'd met. It was sort of… terrifying. Sam and Dean were his best and only friends, and that one of them wouldn't even know him was almost too much to think on.
"He'll… want to know," Cas said eventually, looking down at his coffee. "Hiding things from Dean never does go very well." And wouldn't Castiel just know that more than most?
"But yes, research. I will help as best I can, of course."