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 blockish robot with several front interfaces, including flashing buttons and knobs. He also has two arms, a single wheel that he moves around on, and a microphone. He is described as having impervious armor plating.

Neutral/Pacifist Route
After having the button on his back pressed, Mettaton transforms into Mettaton EX, the final body he commissioned Dr. Alphys to make. In this form he has tall boots and light body armor, along with an overall more humanoid appearance. He strikes poses during the fight, getting 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.

Personality
Mettaton is a confident, and charming TV host, that loves drama, action, and violence. He lives for his ratings, and loved performing, showing a rather shallow appreciation for existence. This however, hides his sincere care for humanity, monster-kind, as well as 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 this all, 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 ingredient 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 Frisk 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's aid, so that he could have a sincere battle against 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. After raising the Ratings of the show (Either by taking damage, using popular brand items, or using specific ACTs) above 10,000, Mettaton stops the battle. To his suprise, 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 possability 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 [LV] 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 you have blown off his arms and legs and get your ratings above 10,000. 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.
 * 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 500 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 100-300 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.

Trivia

 * Mettaton's name holds a close similarity to the Christian Archangel Metatron, although there don't appear to be any similarities between the two.
 * 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.