[Nisei is making an invisible effort not to roll his eyes at you, with your repetitive variations on "what if I'm not who everyone thinks I am?" He knows why you're insecure about that. But asking him how he knows you're a sacrifice is like asking him to prove you're a human and not a rabbit; the "where do I even begin?" is preceded by a resentful stab of "why is this even necessary?"

Though he can see that it's also hard on you, and his annoyance isn't really comparable to your disorientation, empathy has never been one of his qualities. He's faking patience. But he's good at it.]

You ask even more questions than I did. [He grins.] And I don't come from a family, so there was a lot I didn't believe. My parents were outsiders. Anyway, you can - you are doing spell battles. If you weren't a sacrifice, no one could use you as an anchor. The first spell that got through Soubi's guard would finish you, because a perfectly normal amount of restriction could pulp your throat. I've seen the chains wrap around you. I've put them there myself. There's no way you're anything but a sacrifice.

[Nisei has other ways of knowing this, but he's pretty sure any explanation that started with "I can feel your energy. It's unmistakable," would freak you out.]