Mettalphys

Mettalphys is the het ship between Mettaton and Alphys from the Undertale fandom.

Canon
As Mettaton detailed in his diary, he handed out advertisements for a human fanclub in the underground, disappointed when only Alphys showed up. However, the two of them hit it off as they could help each other get what they want: Mettaton wanted a body, Alphys wanted to build a robot to become a royal scientist. Mettaton's new body was a hit with the Underground citizens, netting him the status of a television star, while Asgore was so impressed with the "robot" Alphys build for him that he hired her.

Neutral Route
When the human enters Alphys' lab, she warns them that a robot she build for entertainment purposes went rogue. However, Mettaton bursts through the very thin walls and immediately starts quizzing the human child.

During the quiz, Mettaton attacks the child should it not answer correctly or respond in time, though the lasers merely half the child's health, meaning it can't be killed. Since the questions are about thing the human child would have no way of knowing, Alphys tries to give hints to the child by forming the appropriate letters with her hands. Mettaton notices this and then asks personal questions about her to get back at her. When Mettaton asks the child who Alphys has a crush on, he himself is one of the options, but picking that one results in him brushing it off as ridiculous. Regardless of what the child picks, Mettaton is happy to leave, getting bored of the quiz.

When Mettaton tries to harvest the human's SOUL to bake it into a pie, Alphys calls them, telling Mettaton that human SOULs are not vegan. So the human child has to get the human SOUL-flavored substitute using Alphys' jetpack feature, but regardless if they succeed in retrieving in time, Mettaton admits that Alphys has foiled him yet again.

The third "game" is in the form of a news broadcast with the human having to diffuse multiple bombs disguised as ordinary objects. However, it doesn't matter if the human succeeds, as Alphys hacked Mettaton's systems and diffused the bomb herself.

Lastly, Mettaton performs a musical, while putting the child through a complicated tile puzzle, with a flame wall chasing after them to raise the stakes. Should the human fail, Alphys will disable the flames, should they succeed, Mettaton himself will disable them, already guessing that Alphys was planning to do so.

Alphys then attempts to navigate the human through the CORE, unaware that Mettaton had rearranged it so the usual way would lead to monster encounters that Mettaton hired. It is a lose-lose situation for Alphys for once, as actually listening to her would lead to the human child having to fight monsters, while ignoring her would lead to her doubting herself.

However, when it comes to the human finally getting to challenge Mettaton himself to a fight, Mettaton will reveal that the child was never in any real danger, rather Alphys had laid all of these traps herself, then deliberately made them as harmless as possible so she could take credit for "saving" the child. He proves this by going off-script and locking Alphys out of the room, so she couldn't deactivate him as planned. Then Mettaton plans to kill the child so that he can leave the Underground by himself and become a superstar on the surface. However, Alphys manages to genuinely help the child by telling them of the switch on Mettaton's back, flipping it should make Mettaton vulnerable. However, Mettaton turns into Mettaton Ex, a more humanoid form that Alphys seemingly designed to appear as conventionally attractive. But when the human fights Mettaton, his ratings shoot through the roof, netting him many encouraging calls from viewers and convincing him to stay underground after all. However, he tried himself out during the fight, leading to him deactivating anyway, Alphys finally gets to enter the room and is distraught at seeing him this way. If the human killed him, Alphys will stay with his body. If the human did not, she will be relieved that he only ran out of battery power.

True Pacifist Route
Should the human and Papyrus succeed in hooking Alphys up with Undyne, backtracking to Alphys' lab has the human find Mettaton's body standing on her desk, being repaired. Further progressing into the True Lab has them find a recording of Alphys worrying that the experience of getting his body destroyed might have soured her relationship with Mettaton.

When most of the cast gathers in Asgore's garden for the final battle, Mettaton will reveal himself to be incredibly supportive of Alphys' and Undyne's relationship, though he says that everyone loves a good show, encouraging them to kiss each other.

Fanon
Mettalphys is not very popular in the fandom, most likely because of Alphys' canon pairing with Undyne being so well-received that people won't accept any alternatives.

However, the platonic tag has more than a hundred fics on AO3.

Fandom

 * FAN FICTION