Mettaton is a major character and the fourth boss. Mettaton is a robot with a SOUL, whose body was built by Alphys. He is the sole television star of the Underground. Mettaton poses as an entertainment robot turned human killing robot in Hotland, though he later reveals the truth to the protagonist at the end of the CORE.

Mettaton originally appears as a gray, largely rectangular box with a 4x5 grid of rectangular lights at the top, similar to the mechanism in Snowdin Forest used for Papyrus's Tile Puzzle. The grid of lights can change colors between red, yellow, green, and blue, and Mettaton uses these color changes in place of facial expressions. He has four dials along the bottom of his body, and the bottom, he has a single leg which ends in a wheel. He has two segmented robotic arms which end in white gloves.

After having the switch on his back flipped in the Neutral or True Pacifist Route, Mettaton transforms into Mettaton EX; a new body he specially requested Alphys make for him. In this humanoid form, he has black hair with a long fringe that seems to cover his right eye, pale "skin" and visible metal segments below and above his left eye.