Monster Tinder

[Amaterasu]

Madotsuki

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Protean!

You might wonder, where is the monster in this picture? It's actually the silver substance that forms an armor! Indeed, Proteans are a sort of alien, a metal slime creature capable of creating a symbiotic relationship with a host, forming an armor around their body and protecting them from outside dangers in exchange of the host's body warmth and movement, as Proteans aren't used to the low temperatures of Earth nor can they move around very quickly to do things like finding food. The host can provide both.
Why am I suggesting this for Madotsuki, you may ask? Because she is someone who hates being alone, she prefers non-human company over human one (then again, any monster would probably do for this particular preference) and yet she is withdrawn, with hikkikomodi tendencies. Protean will never give her up, never let her down, never run around and desert her, he'll never make her cry, he'll never say goodbye, never tell a lie and hurt her.
In all seriousness though - Protean will be happy to be with her, and he is as fiercely loyal to his symbiote as Madotsuki is to her friends. He will never leave her side for very long, if at all, and his nature to form into armor at any hint of danger or discomfort will allow Madotsuki to remain withdrawn without staying in a room. If she feels uncomfortable Protean will wrap around her until not a shred of skin remains visible beneath his armor, and if there is danger he will do the same.
Of course, having an alien creature all up in your personal bubble at all times may not work for Madotsuki initially, or at all. As with a lot of characters on this list, she'll need to warm up to Protean first, and Protean won't pick his symbiote based on a first impression either.

Bendy

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Kitsune!

Specifically, a kitsune with two or three tails, not the full nine.
Bendy's trickster, mishievous disposition and his duality of lightheartedness and humor against his tendency to do bad things, as well as his liking to be the one in control, all factor into why I made the decision to partner him with Kitsune. These fox spirits are mishievous tricksters, but not necessarily evil unless they fall under a demonic influence. Bendy may be a spirit as well, which further adds to the points that they have in common.
Kitsunes can have up to nine tails which serve as an indicator of their strength - nine is reserved for the strongest and rarest of them, which are worshipped as gods by some. Two to three tails definitely indicates power, but no exorbitant amount of it. Kitsunes of this range are relatively young (which can still mean dozens of years, so no - Bendy won't get matched with a loli) and more open to external influences, meaning that Kitsune would adapt to match Bendy better. If he's playfully evil, Kitsune will stay the way she is as she grows stronger. If he's actually evil, she'll be corrupted into a demonic kitsune. In either case, by the time that she gains additional tails, if ever, she would be close enough to Bendy to not try and take the reins in their relationship even if she would have the power to. Many Kitsunes enjoy someone leading them, in fact - there's no point yelling about patriarchy and female dependancy on men here.
Finally, many kitsunes are skilled with a paintbrush, using their tails as one. Perhaps she can help Bendy in some way using this skill, be it improving him or drawing a few of his friends? Who knows.