Monster Tinder

[Amaterasu]

Samus Aran

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Centaur!

Samus didn't have a lot to go on in terms of relationships, outside perhaps of the Metroid Infant. And that's more a mother/kid bond than a romantic one... so, let's look a little deeper. Samus is a Bounty hunter who, for being called that way, works very close with the Galactic Federation. She lets them analyze her technology and they support her in many ways. She fights the Space Pirates who are a threat to the Galaxy and she frequently endangers her own life to save survivors of disasters or to exterminate dangers such as the Metroids and the X Parasite. She is very much a "knight" sort of character, with a strong code of honor, morals and the will to do good. She may be a little "rough" like a soldier, and she respects strength and valor. Should she be matched to a scrawny nerd or a cute gal? No, I say.
Of course, Centaur would not exactly be on the same level as her regarding technology and such, but that's no unsurmountable obstacle - and really there's few monsters besides Space Pirates who are familiar with science-fiction technology, and it's doubtful that Samus would ever get friendly with a Space Pirate. However, there are few monsters who have codes of honor as strict and who are as proud as centaurs: you'd better write your testament before you try getting on the back of one. They're also not easy to get friendly with, but Samus will likely not approach a centaur with a flirtateous smile and her Zero suit on after all. They'll probably work together. And slowly get to know each other. And realize how much they have in common. And it'll evolve from mutual respect and a "bromance" into... well, more.
Furthermore, Centaurs are strong. They're warriors without equal, they are cavalry without the need for a horse, they are bowmen as feared as elves and their massive, long spears penetrate any opponent that gets too close... both kinds of spears ( ͡° ͜ʖ ͡°) Samus can respect that. And she needs someone strong with her, someone she can trust in, both in battle and in privacy. A Centaur would be a great fit.

Enel

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Wight! (Undead)

Oh boy, Enel... someone with a god complex, the powers to control lightning and a machine to make that power strong enough to bombard an entire country? Not to mention, he's a rather volatile individual. Pairing him with a "smart wife" type of gal who is a little manipulative behind the scenes and whispers advice into his ear wouldn't work either, because of his mind-reading powers. He's not above using them on his partner, I'm sure.
So, the girl that stands up to Enel must be someone who can take his lightning without a problem! Wight, being an undead, doesn't have all those issues that living beings would have such as a brain being overloaded by lightning impulses or their body getting fried by energy. Well, Enel might not like that initially, being used to being almighty and whatnot. But once he warms up to her a little, that won't be as big a problem anymore. And boy, WILL he warm up to her, I don't doubt it... Wights are cunning, tricky and above all power-hungry undead, of great intellect and lewd as a bonus. She's not going to be contradicting him at every turn, telling him that his great plans are garbage. She's going to help him improve them, to dominate the world if it needs be, while at the same time keeping him grounded in reality. Without needing to worry about his lightning she can afford to be more honest to him than others, and once Enel realizes that that brain in her noggin is functioning very well for someone long dead he'll probably start listening. And Wight brings a lot to the table: she can turn others undead, which combined with Enel's lightning will create quite the army. Her dark powers complement his lightning techniques, and if against her advice he decides to go to the moon... well, she doesn't need air. But if they stay on Earth, be afraid, humankind. Be very afraid...