[He's genuinely surprised that Ayaka hadn't divulged any details to Ayato about himself and Yoimiya. They were siblings, and that seems like to him the kind of information that would be shared especially considering he's Fatui. But he's going to have to take the L on this one and admit that maybe he assumed a bit too much here. He's just so used to people reporting on the Fatui's movements to the point where it would be used as info against him or at least a warning...but he guesses not here.]

First, I'd like to say thank you. Having Yoimiya in my life has been a wonderful, unexpected joy. And second...

[He thinks over Ayato's words. Guess in this case, they both have something in common here. If it were him, and his little sister was here, he'd be doing a lot too to make sure his sister was safe.]

I know Yoimiya is well equipped to take on the many challenges this world presents to us, but I still can't help but worry. So, you have my thanks again for watching over her on the quest you both embarked on. That means a lot to me.