Monster Tinder

[Amaterasu]

Desman Black

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Slime!

(Finally picking things back up here)

Desman's big problem, as explained by him, is that he blames himself for the deaths of those around him - and everyone around him seems to die for some reason as if it was fate. It is usually not his fault nor could he have prevented it but it drove him into a very socially withdrawn state and even made him give up his humanity. Now I know that I said that a good monster partner should match the character as they are, not forcing a change on them, but in this case it's different.
Slimes are among the more enigmatic monsters, and that's saying something. Some slimes have "cores" that form the organic, intelligent part of their otherwise liquid body, others don't - and this particular one belongs to the latter class. Each cell in her body is a deposit of thick liquid, capable of linking itself to other cells to establish an intelligence, whilst serving as a memory storage in and of itself - the conglomerate of cells shapes itself into a humanoid form, which is then a slime. Effectively, her body is immune to just about any form of harm - physical projectiles or weapons would simply pass through, whereas magical and other supernatural attacks have next to nothing to "grasp" at because you're attacking billions of single cells, not a single organism. That doesn't make her invincible, in fact her big weakness is a very common one: namely water. In all its forms it poses a threat to Slime, who could dissolve in a big body of water or absorb rainwater to give two examples, and lose the necessary consistency to keep herself in shape. Whether or not she can really die this way is unclear. Furthermore slimes are, while not dumb, only childishly intelligent at first and require the support and guidance of others to accomplish more complex tasks. They learn over time.
In other words Desman is not going to have an independant immortal girlfriend that he needs not to worry about ever because nothing will kill her - Slime would require constant care, in the event of rain he'd better have an umbrella ready to bring her somewhere dry. Slimes have another notable feature however, it's that their personality, much like their body, is very malleable. They learn by mimicking those around her and develop a personality based on that, thus Slime would rapidly become a useful ally if Desman just invests some time into her, and she'll wisen up to the dangers that surround her. Worst case scenario, she gets "killed"? She could give Desman a piece of herself to keep in a testing tube as a backup of sorts, though her loss would probably still feel bad. Not as bad as completely losing her. The backup can reform into Slime.
In short, Desman would keep losing Slime to various circumstances, then regain her by letting her backup out of the test tube. He would learn that by caring for her and by being safe he can save her. That maybe, fate is not absolute. He's not safe from losing her, but she won't disappear from one day to another. And Slime's childish, fun and yet friendly and caring personality (that's what most slimes develop) would remind him that there's fun in being with others.