[Oh. That's new. Especially since his labcoat and battle body cover his ribs, but, well, she must just be curious! Maybe she's not used to skeletons? While he blinks a few times and stares a little, he's back to his smiling self before long. After all, interest is a good thing! And he can understand the surprise in seeing a new and unusual being. Humans were strange to him at first, too!

Then Frisk happened.]

Oh! Well, they just sort of... do. It's a skeleton thing!

[He says this without hesitation, like it's perfectly normal and there's nothing strange about this.]

Besides, if my bones were in pieces, I'd have a hard time crafting perilous puzzles. Or cooking succulent spaghetti! Or performing any number of scientific feats.