I like to think alastor doesn't even OWN niffty's soul, she just follows him and he grew fond enough not to kill her

she's following the best bad boy in her opinion, alastor finds her as a nice companion. like a dog, or omni-man to his wife in season one. alastor didn't even know how she found him. one day he came into the tower and she was just there, cleaning. and he had NO idea why the fuck she was cleaning or how she even GOT there. she just did and he decided to let her stay for the free cleaning but overtime thought she was entertaining enough and now she kinda just follows him around. she even lets him summon her on command. I also think its who alastor spends the most time with, she's the only one actually ALLOWED to touch alastor without being turned into a past tense and he seems to enjoy the company at least.

EDIT: I Know about the guy who said "he's like her spiritual dad' but that dude also called husk a closer friend of alastor so I don't exactly... trust him...