If we're going by the assumption that everyone is ok with you marrying Kokushibou, then:
Venue:
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Of course, he is still a demon, so it would either be inside or in the dark. I think it would be somewhere like in the mountains or the forest.
Dress:
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Teeny tiny stars!
Hair:
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Matching hairstyles <3
Wedding Band:
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Like come one, it's just too cute not to.
Lingerie
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He's just a purple kind of person. He also likes that bit of modesty.
Who Made The First Move
I don't think either of you can really tell, it just kind of happened. But if I had to choose, I'd say probably you made a couple of comments and he began to pursue it harder.
The Thing They Like Most About You
Never have you ever looked at him like he was a monster, even when he really knows he is. You just love him unconditionally.
Break up
"How long have you been fraternizing with Demon Slayers, Kokushibou?"
He sort of knew it would happen, deep down, but he had hoped he had at least a little longer with you. Standing before Muzan, he said nothing. He felt his legs sweep from under him, forcing him to kneel before the other man.
"Answer me!"
He took a short breath to ground himself, "I-I have only come into contact with one."
Muzan let out a humourless laugh, "But, she is a demon slayer, a hashira, who is in contact with other slayers. Now. How. Long?"
He knew better than to lie or not answer his master,"...7 months" he quickly rose to his feet, "Please, she's harmless, she makes me remember things"
"You mean she is harmless to YOU. She is still a demon slayer. It's her job to exterminate us. You remembering things is not an improvement, it's a hindrance"
There was silence once again for a few moments, the tension rising. Kokushibou has a sickening feeling of what was coming next.
"Upper moon one. Go to her. Go to her and tell her you love her"
Now that, he did not expect.
"And then leave her." He knew it was too good to be true. He felt his heart sink.
"Please, no! She- she's all i have"
"YOU HAVE RESPONSIBILITIES!" the overlord roared, "And if you dont do it, I'll kill her"
~~~
So he did. He didn't have any choice. He met you the next night and he told you he loved you, and then, he told you he couldn't see you anymore.
Him Without You
He felt like hell on earth. He really wasn't sure what to do anymore and everyone seemed to notice his demeanour had changed.
He kind of lost motivation to keep going after living so long. He'd been close to just walking into the sun multiple times.
