You'd been taken in by Bobby years ago after running away from your parents, who- well, calling them abusive would be a kind word for it. Since then you'd lived with Bobby and become like a younger sibling to Sam and Dean. Though, since running away, you hadn't spoken a word and communicated through sign language.

And now they had an angel that followed them everywhere.

"Dean did tell me you don't speak. I see now that that is true," Cas said to you. "I assumed he just meant you were quiet. I can understand you just fine, as an angel I understand all languages, even the nonverbal ones. And you can understand me, I presume? You're not hard of hearing?"