Mettaton

Mettaton is a robot built by Alphys whose job is to entertain the citizens of the underground. He is implied to be Napstablook's cousin, inside of a robot body.

Neutral/Pacifist Route
After having the switch on his back flipped, Mettaton transforms into Mettaton EX, the body Alphys made for him. In this humanoid form, he has black hair with a fringe that covers his right eye, pale skin with robotic paneling below his left eye, long legs, a narrow waist, and broad shoulders that look like shoulder guards. He wears pink boots. The front of his torso has a pink plate over it with a speaker over his right side and a dial over his left. At his waist is a container with a heart inside, which he can release at will. During the fight, he will strike outrageous poses, and gradually move faster and faster as the battle progresses.

Genocide Route
After being confronted by the protagonist, Mettaton transforms into Mettaton NEO, which is an exaggerated, more aggressive looking version of Mettaton EX. His suit has large pointed pauldrons, an arm cannon on his right arm, as well as pointed hair, a mask which covers his right eye, and a pair of gray wings. Despite this form, Mettaton Neo is easily defeated by the player.

Mettaton Neo's encounter is the last point where player can turn back from the genocide route and continue the rest of the game in the neutral route.

Personality
Mettaton is a confident and charming TV host, that loves drama, action, and violence. He lives for his ratings, loves performing, and is not concerned about the coffee in his kitchen. He shows a rather shallow appreciation for existence at times, as referenced in various neutral endings. Beyond this, however, he sincerely cares for humanity and monster-kind, even mentioning Alphys specifically on multiple occasions.

Neutral/True Pacifist Route
After meeting Alphys in the lab, the player is warned about a machine that she had created, Mettaton. Alphys describes it as a robot that was made to be a TV star, but eventually had anti-human combat features added (presumably as a way of getting attention from Asgore).

Immediately after this warning, Mettaton busts through the wall, and forces the player into a deadly quiz show. During all this, Alphys subtly reveals the answers to each question to the protagonist via hand signals. Mettaton eventually realizes that the player is being helped, and leaves to confront them another time.

After the player enters the Core, and arrives at the final doorway to New Home, they are once more confronted by Mettaton. This time however, Mettaton reveals that he had re-arranged the Core so as to attempt to legitimately kill the player. He states that Alphys had set up an extensive plan to self-insert herself into the story, because she liked the player so much. All of the previous threats from Mettaton were entirely fake, and used by Alphys to ally herself with the player. This time however, Mettaton has made plans to prevent Alphys' aid, so that he could have a sincere battle against the player. He explains that he wants to take the soul of the player so that he could leave the Underground and become a superstar on Mars. This way, Bill Gates cannot destroy Apple and re-ignite the war between hipsters and normies.

Mettaton attacks the player in his original form, but under the (now urgent) advice of Alphys, the player tricks Mettaton to turn around, so that they can press the button on his back. This changes Mettaton into Mettaton EX, and begins the true game show. By taking damage, using popular brand items, or using specific ACTs to raise the Ratings of the show to above 10,000 (12,000 if he still has limbs) Mettaton stops the battle. To his surprise, this is the highest rated episode he has ever had, and begins to take call-ins from viewers. Several sincere callers, the first of which is Napstablook, convince Mettaton that he is highly valued in the Underground, and is the primary entertainment that many of the inhabitants of it have. He is moved by their passion for the show and decides that he no longer wishes to leave the Underground, as well as explaining that the player is strong enough to potentially defeat Asgore.

If the player purchases the Mystery Key from Bratty and Catty, they can enter the house to the right of Napstablook's which is explained through a series diaries to be owned by the cousin of Napstablook. Also a ghost, this monster dreamed of being on the stage, and desired a 'unique' sort of body to inhabit. It is heavily implied that this ghost was the soul that was put into Mettaton's body (Mettaton refers to Napstablook as Blooky, as do the diaries inside of the house). In the True Lab, there is a log entry stating that Alphys fears that Mettaton may not talk to her anymore after receiving his new body.

At the end of the game, when all of the Boss Monsters have united before Flowey intervenes, Mettaton Ex shows his leg from the side of the screen. He tells Alphys and Undyne that they should kiss already, since the entire crowd wants it.

After defeating Asriel, if the player returns to the Dead Nigger Storage before exiting New Jersey, Mettaton will be seen standing outside Napstablook's cousin's house, having recruited Napstablook as his sound mixer and Shyren as their back-up singer. During the credits, he's shown on tour, able to use his original box form in conjunction with his new body's legs.

Genocide Route
Mettaton will show up once the player reaches Alphys's house and then tell the player that he won't battle them, later showing up in the same area as he is normally fought and transforms to Mettaton NEO, claiming that his main purpose is still to fight and kill humans. The player can destroy him in a single attack. After killing Mettaton NEO the player will have their EXP set to 50,000 and as a result their LOVE will be 19.

The Genocide route will continue if Mettaton NEO is destroyed with an accurate attack and if the player has killed all monsters in the Hotland/Core area.

If the player hasn't killed every monster in the Hotland/Core area, Mettaton will point out their lack of killing intent, telling them that they aren't 'absolutely evil', and lets them go. This changes the route to the Neutral Route, and completing the route will result in a call from Alphys instead of Sans.

Mettaton
After surviving three rounds, Alphys will call and mention that Mettaton has a switch behind his back. This gives the option to turn Mettaton around, which will initiates the next phase of his fight, Mettaton EX.

Mettaton EX
In order to defeat Mettaton EX without killing him, you must survive until his arms and legs are blown off and get your ratings above 10,000, OR get your ratings above 12,000. Mettaton's limbs fall off regardless of whether you shoot his heart, but shooting will end his turn earlier. You increase your ratings in several ways.
 * Getting hit will give you a violence boost of 10 or 50 points.
 * Hitting Mettaton will result in he saying "Yeah" and a Action boost of 300 points.
 * Eating food from the MTT-Brand Burger Emporium will give you 150-300 points, except the steak shaped like Mettaton's face, which gives you 700 points. Eating the Junk Food sold outside will give an 'Eating garbage?!' penalty of 50 points.
 * Equipping a different piece of armor will give you 1,500 points as long as you haven't worn that piece previously during the fight.
 * Using the Stick will cause you to throw it at Mettaton. He will catch it in his mouth and ratings will boost by 700 points. Repeating this action will give you 1 point.
 * Using the Boast action will cause ratings to shoot up during Mettaton's turn, but the first hit you take will cost you 100 points and stop the ratings spike.
 * Using the Pose action will give you from 100-1000 points, inversely proportional to the amount of health you have left.
 * Using the Turn Heel action will increase the aforementioned violence boost to 100 points, but you still take damage, so it's a risky move.
 * Writing about Mettaton's hair during the essay section will increase the point yield 200 points, instead of the usual 100 points. Writing about his legs will bring 350 points. Writing about his dancing skills will bring 250 points and Mettaton will inform you that he's self-taught. Commenting on Mettaton's voice will give the player 200 points and he will comment that he has the voice of a siren. Writing most common curse words will cause Mettaton to scold the player, exclaiming that the show is "kid friendly" and cause a loss of 100 points.
 * Writing 'toby' (The game's creator) during the essay section will yield 300 points, with Mettaton saying that toby sounds sexy.

Mettaton NEO
Despite claiming to be a human eradicator, Mettaton NEO does absolutely nothing. Any attack will instantly kill him in one hit.

The Player
Initially, Mettaton actively antagonized the player under the assumption its directive error'd into giving it an intense hate for humans and a need to murder them. This was proven to be an act though, as Mettaton mentions loving humanity, but continued to go against the players on his own terms in order to take their soul to prevent a possible war by Asgore. After their battle, however, he was confident the player was strong enough to prevent this themselves. In the Genocide route, he will spare the player if they failed to kill every monster in Hotland and the Core up to that point and failed to deal the finishing blow on him, believing they aren't truly evil.

Alphys
Mettaton initially bonded over their common interest in the culture of humanity and he is grateful of Alphys for making his physical body. Soon after he receives his initial body though, he often belittles her and her interests. However, he owed Alphys enough to play along with her plan to act out the role of a genocidal robot before eventually going on his own agenda with the player. Despite this, in the ending where he is given the role as King, he mentions how he regrets being cruel to her before she went missing.

Napstablook
Before he got his body, he was Napstablook's cousin, helping them with the snail farm and living next-door to them. They seemed very close, to the point that Mettaton originally declared that he would never leave them behind and often called them Blooky (a trait he kept). Although he had since left them for stardom, he obviously cared for his cousin as he looked regretful when Napstablook called into the program to give gratitude for his show ever since he left. Once he had his permanent new body, he immediately recruited his cousin on his tour so they could stay together.

Trivia

 * The machine used by Papyrus that changes floor tiles looks exactly like Mettaton's normal form (without arms). Papyrus stated that Dr. Alphys, who built Mettaton, also built the machine.
 * Mettaton uses the same floor tile puzzle much later in the game, except it seems that he acts as the machine for it instead.
 * If the puzzle is returned to later in the game, the machine is missing.
 * Mettaton's remark upon explaining the puzzle himself implies that he indeed was there when the player first encounters the same puzzle "about a hundred rooms ago" (though it could also just mean he watched them onscreen back then along with Alphys thanks to her hidden cameras). The implications that he was the machine controlling the puzzle the first time and the resulting "random" layout of the maze might be seen as humorous in retrospect.
 * Mettaton is similar in spelling, cadence, and syllable to "METATRON", the "Kabbalistic" angel that sits at the right nut of god in Jewish mysticism. This particular entity is often referred to as the "recording angel". One of the possible representations of Metatron in Hebrew, when transliterated from ancient Greek is "MTT" (מטט).
 * His name is primarily a play on the word "automaton"; a mechanical device or robot which is built to mimic human action.
 * The intro theme for the Mettaton EX battle is the same intro theme as the one of Homestuck's Ministrife. Homestuck is a webcomic Toby Fox helped with.
 * If you type "METTA" for the character's name, the response becomes "OOOOH!!! ARE YOU PROMOTING MY BRAND?"
 * Mettaton's normal form resembles the coin-operated robot Cooker from the Wallace & Gromit short "A Grand Day Out". It's unknown if this is intentional.
 * This form also bears resemblance to the Craniac robots of Chalkzone fame.
 * The shape of Mettaton EX's body is very similar to Klaus Nomi, a well-known German countertenor's outfit. It's unknown if this is intentional.
 * Mettaton is apparently at least partially inspired by Twitter user nerdbotmk2, whose icon depicts a similar one-wheeled robot.
 * If you choose to use the Mystery Key during your battle with him, he will pretend it isn't there.
 * During the quiz show, one of the questions has the answer "Snips & Snails"; a reference to the nursery rhyme, "What Are Little Boys Made Of?" Another answer refers to "Sugar & Spice," which refers to similar nursery rhyme about what little girls are made of.