For Frau, he was talking about actual light. That's what Teito was for him, a real light. The path that Frau walked was pitch black and only getting darker. Teito was the only light present on that path, a light to guide him back.

He sighed heavily when Joe mentioned being used and couldn't help dropping his elbow onto the table and then his chin into his hand.

"What is it with you kids and that? Or what the hell was going on when you were all born? It wasn't even the same world..."

They were certainly close enough to Teito's age, and the kid had been used as a puppet as well. It was a pain.