Papyton

Papyton is the slash ship between Papyrus and Mettaton from the Undertale and Deltarune fandoms.

Canon
Mettaton doesn't seem to know anything about Papyrus even if the skeleton is one of his biggest fans. However, Papyrus watches his shows. Papyrus is an hudge fan of Mettaton. If you call him while you're at Hotland, he will describe Mettaton as his favorite sexy rectangle. He also says that he wishes he could live in there to see his idol, even if he said multiple times he was glad he didn't live in this region. He also pretends that Mettaton's ghost is called Happstablook, the happy ghost, even if Undyne doesn't think so.

If you go in Papyrus' bedroom, you can find figurines of robots that could be representing Mettaton. The skeleton also comments on how beautiful his eyes are. Before Flowey captures all the souls in the Pacifist ending, Papyrus will briefly see Mettaton's leg and look impressed. In an Neutral ending, if Mettaton becames the king and Papyrus is still alive, he becomes his agent, with Sans. Otherwise, if Mettaton dies in another Neutral ending, Papyrus never talk about it in a phone talk.

These two characters are similar, as they both appear arrogant and flamboyant in their first respective appearances. However, they both have more than meet the eyes. Papyrus, being a lovable goofball, and Mettaton being a former ghost who lost himself in his ambitions. Both see the errors of their ways thanks to the Player.

Fanon
People like to pair them because the both have a lot of self-confidence and care a lot about their friends. Some people also say that Papyrus is the only other monster in the Underground with an ego big enough to keep up with Mettaton's.

Most of Papyton comics are representing Mettaton trying to seduce Papyrus, despite him being a really innocent person. In some of them, there is also Sans who, obviously, disagrees with this ship. In some, Mettaton is portrayed as a woman and they have children.

A certain comic have the two meeting before, where Papyrus inspires Mettaton before he became who he is now. Happstablook was singing "Haven't You Noticed (I'm a Star)" from Steven Universe. They were surprised by Papyrus and tried to fade. But Papyrus, who genuinly complimented the ghost, encouraged them to follow their dream. The last sentence was "Thank you, Papyrus."

Fandom
FAN FICTIONS

Mettaton/Papyrus on AO3

Trivia

 * Mettaton is often mistaken for girl, as he crossdresses and wears pink in his EX form.