Some of the other characters have traits belonging to people she knows, her sister(i believe Nikki is the one that ties closer to her sister) and others.Afaik. and have read, Valeria is Nyx alter ego, that could mean a lot of things, I'm shure that more of the LI have (like in all VNS) perks and personality traits of the author.
Valeria ties closest to Nyx herself, she admitted as much, her nickname here used to be Nyx-Valeria and she used a Val avatar.
Other than that a variety of things can be used as inspiration, people/events you were told about or even movies/books.
Nyx's "thing" for best friends getting in conflict over a guy must come from somewhere... maybe she loves it in movies/books, maybe she witnessed a couple of times with friends, maybe she was part of one such conflicts...
I'd love to know the answer to that lol.