Mettaton is a four-armed, soul-powered robot designed by Alphys and occupied by Napstablook's cousin. Speculatively, some say their canon name was "Happstablook" due to a phone call in Undertale where Papyrus refers to this name as his headcanon for Napstablook's cousin before becoming Mettaton.
Mettaton's normal/box form resembles his original Undertale counterpart, the difference being his two pair of arms, the cracks across his black-colored body and the red "X" on his screen. His gloves are red (on his upper arms) and yellow (on his lower arms).
Mettaton's EX form, however, is much different from his Undertale counterpart. He has a red chestplate, red boots, four arms (just like in his normal form) and four red eyes (only two being shown, the others being covered by hair). His soul can be seen under his chest, as a "Cracked Soul."
Unlike in Undertale, where Alphys and Napstablook's cousin worked together to produce a robot body that aesthetically pleased the ghost that would soon inhabit it, in Underfell, Alphys has possibly forced the ghost inside of her creation, an effort to experiment in creating a killing machine that may not require any source of fuel or power and instead runs on the soul of a monster. She does not allow the ghost to leave the robot. She potentially uses painful means of punishment if Mettaton does not comply with her orders, or fails to perform in a satisfactory way. It's heavily implied that Mettaton is abused by her and doesn't want anyone to get hurt at his hands, though he is forced to attack at Alphys' command. Because of this, Mettaton is commonly seen using the quote "I never asked to be made!"
His personality is said to be a mix of dry humor and melancholy retorts. Unlike in Undertale, Mettaton does not relish showing off his EX form, and instead of emitting a glorious "Oh, yes!", he sobs a painful "Oh, no..." before battle. He believes himself to be incredibly ugly but still holds the dream of becoming a beloved superstar.
Alphys: It is implied that Alphys is Mettaton's abusive owner, torturing him if he doesn't listen to her. It is sometimes said that he was forced into the robotic body by her.
Napstablook: Napstablook is Mettaton's cousin, but they hate Mettaton for leaving them, and even calls him during his show to tell him to self-destruct.