Mettaton

Mettaton is a robot created by Alphys as a "robotic TV star." He is first met in the Lab.

Appearances
Mettaton in his original state is a rectangular robot with several front interfaces, including flashing buttons and knobs. He has two arms, a single wheel that he moves around on, and he's often holding a microphone. Mettaton can swap out his wheel for a rocket jet, which he can use to hover in place or fly away. He is described as having impervious armor plating.

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, and loves performing, showing a rather shallow appreciation for existence. Beyond this, however, is his sincere care for humanity and monster-kind, many times mentioning Alphys specifically.

Neutral/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 leaving the Lab, Mettaton is encountered several times throughout the Hotland, including a deadly cooking show (Where the player must bring several ingredients to the counter), a news report (Where all the items of importance are revealed to be bombs, that the player must disarm), and finally a deadly puzzle (In fact a functioning version of the colored-tile puzzle that Papyrus attempted to present the player with previously in the game). In each of these scenarios, unique functions that Alphys has programmed into the player's Cell Phone as well as phoned-in advice/hacking skills from Alphys herself bails the player out of the situation. With each progressive encounter, Mettaton becomes less and less enthused, revealing his disinterest in attacking the player.

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 the Surface. This way, Asgore cannot destroy the Barrier and re-ignite the war between humans and monsters.

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 he 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 being 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 Mettaton may not talk to Alphys anymore once she gives him his body.

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

After defeating Asriel, if the player returns to Napstablook's house before exiting the ruins, Mettaton will be seen standing outside Napstablook's cousin's house, further enhancing the possibility of Mettaton's soul being Napstablook's cousin.

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 misses their attack, or 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.

Battle
In order to defeat Mettaton 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.
 * 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 not give you additional points.
 * 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 100-500 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 to 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.

Trivia

 * The machine used by Papyrus that changes floor tiles looks exactly like Mettaton's normal form (sans 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.
 * Mettaton is similar in spelling, cadence, and syllable to "METATRON", the "Kabbalistic" angel that sits at the right hand 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).
 * 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.
 * In his first appearance Mettaton crashes through a wall like the Kool-Aid Man, with a slightly different catch phrase. It's unknown if this is intentional but it's funny to think about.
 * 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?"