He knew he was sane when he heard voices in his head. Mordion tried to laugh at the irony, but it wasn't really funny.
"Slave? Are you all right?" he could hear her chirping like a fearless sparrow. "Don't despair - you'll show them all someday," was one of her favorites, followed by, "You know we're here for you."
But it wasn't her optimism that saved him. She had no idea what it was like to be him. She knew of course that horrible things had happened to him. He carefully avoided giving her details, but he was afraid he had told her just enough to let her romanticize it. Regret and relief fought over that one. At any rate, the fathomless murk of his life, in which hope dangled like a flimsy bauble, was impossible to explain to another soul. Her unshakeable belief that Things Would Be Better Someday was as meaningless to him as it would be to preach universal flight to a fish.
No, it wasn't that she gave him hope. It was that even in thought, she treated him like another human being.
Even better than her encouragement, he liked when she asked them all for advice about her life. She had no idea how ridiculous the conundrums of her privileged life seemed, and that was beautiful. He loved to be asked whether she should sneak out to go riding with her cousin despite the fact that she was supposed to be studying. When she ranted to them all about how unfair it was that she was kept home in punishment, he practically basked.
For a long time, he assumed that if she could see who he was, in reality, she would shrink like everyone else. Sometimes he dropped hints. He let her see glimpses of how broken and twisted his mind was. She told him no one could ever blame him. He heavily implied that he had killed people. She sympathized.
When he realized that there was one person who treated him just like that face to face, and that she and the Girl Child might be one and the same, he knew he was lost.