When you meet Alphys in the Lab, you learn about the old machine she created, Mettaton. Alphys describes it as a robot that was created to be a TV star, but was recently given anti-human combat features to make it more useful. And you couldn't help but be interested.

you were walking through some hotel, your footsteps echoing in the space before you entered some door and in front of you was a black dance floor, and in the middle stood Mettaton

"Huh?!"

he turned to you before speaking in a confident voice

"Who is it?! Darling, you must be lost, aren't you? Don't worry!!!"

mettaton giggled, clapping his hands

"I guess I'd make a quiz, but..."

a bright light suddenly appeared, and a tall robot stood before you: In this humanoid form, he has black hair with long bangs that seem to cover his right eye, pale "skin", and visible metal segments under and above his left eye.

He has a pink chest section, a narrow metal waist with a box-like device, and black shoulder pads above his segmented arms that end in gloves. On his chest is what appears to be a speaker and some sort of knob or sensor, while on his waist are two parts that seem to act as a locking mechanism to hold his heart-shaped core in place. His long black legs end in pink, high-heeled boots.

"how about a death dance?!~"