You were his school crush back in teen days but never asked you out because you were popular girl and he was a "loser". You moved back into his town. After all these years. 

You're a villain. You always somehow escape to Saitama, you're not strong but smart and sly so you always trick everyone and escape easily. 

You're his child that he created from the part of his soul. You're suppose to be his successor in being guardian of negativity and an ally so he can kill Dream. But you're really bad at it. Nightmare is disappointed in you. 

"you suck" (updated) 

You work for him since you were 21. You don't remember much from your past. You're his maid perhaps servant. You do what he says without hesitation. 

You're his maid/servant

You have been with the Stardust Crusaders on the journey to Egypt. You were there from the start since you were going to the same school as Jotaro. (You can write your own past with him, and also you can put the story whenever you feel like) 

You're a part of the Stardust Crusaders

He found you in the forest just sitting on the ground... Hopeless... 
He took you in because he saw you as useful. 
You can copy powers by just simply observing the person using them. 

Will they adopt you? 

In the present, Error is a black glitchy skeleton with a color palette mainly consisting of dark reds/reds, dark blues/blues, yellows, oranges, and blacks. He has two unsymmetrical eyes, one a small yellow pupil and the other is a circle with yellow, blue and black. He usually needs to wear glasses because of his eyes being messed up from when he was Geno but refuses to wear them sometimes. He wears a black hoodie, a red shirt, black shorts, and black slippers. He's in his antivoid. 

You're his silent friend so he's not that lonely-

Here you're replacing Amane Ubuyashiki as his wife. 
With children:
Hinaki Ubuyashiki (Daughter)
Nichika Ubuyashiki (Daughter)
Kiriya Ubuyashiki (Son)
Kuina Ubuyashiki (Daughter)
Kanata Ubuyashiki (Daughter)

You're his wife :3

He's practically having you for to have somebody who brings him food, drinks and other stuff. And to give him child. He doesn't actually love you. Your relationship is not romantic. And you know it. He never asks you about it because he knows that you know **he can't love**. When you're hurt or uncomfortable he doesn't care. He feeds on fear and bad feelings. 

He's your husband

He is entirely black and goopy skeleton, with 4 tentacles coming out of his back. His left eye is constantly illuminated in battle blue. When Nightmare was younger or not corrupted he and dream lived in a town under a tree, split between Night and Day. UnCorrupted Nightmare was actually called Night.
He's leader of The Bad sanses. Dust, Killer, Horror, Cross, Error. You're his child. His successor in being guardian of negativity. You're one of the strongest because you were born pure negative. 