[Sypha looks enraptured, but more like she's watching a train crash in slow motion. It's like one of her people's stories, passed down from the future and another world and into her hands. She's piecing together the terminology. "Fire Nation" is a bit on the nose, but if they can use fire magic (and presumably only fire magic) then it's fitting enough. And this situation is eerily familiar, down to the walking in the front door of the castle and the killing of family and the fate of the world as they knew it.

She's been there. She knows this too well.]

So you and your revolt can't fail. And you need to storm your own family's castle to put an end to this Fire Lord, or he'll be unstoppable.

And...you're here.

[She's trying to put this tactfully, but it's kind of hard when the stakes are this high for the boy.]

If you're a firebender yourself, why does it need to be you?