Mettaton

Mettaton is a main character in Underfell. He is a robot built by Alphys and the beloved star of the Underground. During Frisk's journey he acts as if he is their "good friend", however he would later attempt to betray them.

Normal Form
In his normal form, Mettaton appears as a curved metal box. He has two arms with his left hand donning a red glove and a yellow one for his right. He has black shoulder pads, three dials and the bottom of the body and a bow at the bottom of his wheel.

Mettaton EX
In his EX form, Mettaton takes on a much more humanoid appearance. He has greasy black hair, two pairs of eyes, sharp hips, latex boots, a cracked red chest piece, and a mouth which is permanently locked in a menacing smile. His defining feature is his two sets of arms, his first pair wears red latex gloves while his second pair wears yellow lab gloves that are controlled by Alphys.

Trust Route
After the Trust Route, Mettaton takes on an appearance much more closely resembling that of his Undertale counterpart. He retains his two pairs of eyes, and now sports a dark magenta color in place of his previous red. His mouth is no longer locked into a smile, and he now has only one pair of arms. His left arm's glove is an even darker shade of magenta, while his right arm's glove is a very faded shade of yellow.

Personality
While Mettaton expresses confidence in his appearance, this is nothing more than a forced program by Alphys. In reality, Mettaton is very disappointed in how his body looks. He occasionally malfunctions which allows him to temporarily break free from the forced programming. He is hilariously overdramatic, often swooning and using the phrase "teehee". He also has zero subtlety when it comes to expressing disgust and will do so very plainly.

Trivia

 * Mettaton's opening line is ‘DARRRLIINGGG~’ instead of ‘OH YES’.
 * Mettaton does not like to be stared at for too long, if you do it will "get dark real fast".
 * Mettaton has the tallest heals in the Underground.