"It's Mettaton." There are no terms of endearment for this child, not even sarcastic ones. "I'd say it's a shame we didn't get to know each other better, but it really isn't. What I will say is this: I can only hope you'll be as brief a presence in my timeline as I was in yours."

And then he changes--but not into his EX form. Oh, no. His beloved body just doesn't send the right message. Never mind that this other body is only something Alphys built for him for a movie role. This child doesn't know that, and he doesn't intend to tell them.

It's all about putting on the right show.