Okay! She smiled! Good. A happy Undyne was preferable to an angry, shouting Undyne. Especially after what Napstablook told him before he and Frisk got whisked away to Vaikuntha. But... it was still Undyne. Asking about Frisk.

He sighed, folding his arms and ducking his head. "UNDYNE, WITH ALL DUE RESPECT, I'M NOT SURE I SHOULD TELL YOU. ON ACCOUNT OF... WELL. MURDERY TYPE THINGS THAT DON'T EVEN MATTER HERE."

He tried to phrase it lightly. Undyne had been through a lot, and he didn't want their reunion to sour, but... Frisk mattered. And now there were two.