9S lets out a long drawn whine. Aw, come on! He's going to find out soon, so why not tell him? But he doesn't say that aloud, instead choosing to say, "I don't have a mother."

And what's a grave, anyway?

"Alright, alright, fine. I swear on my duty to serve mankind," even though he's technically abandoning that. Does it count if he's running away to be with another (mostly) human and help her change her world? "that if I find anything weird in your arm and eye that I'll keep it between us."

Unless it's something that he thinks someone else really, really needs to know, anyway.