Should I make a sia_(quilt) tag to differentiate between the character and the artist? It might be better if I knew Sia's last name. I'm not even sure if she has one. And as for Airi would someone mind confirming that her last name is Kahara?
I think you should make a unique tag for this character from Quilt.
I can't read the Japanese, but from the artbook I own it seems that Sia is a demi-goddess of the sea of some sort (how else could she let Takumi breathe underwater just by touching his lips with her tongue?). So I don't think she has a last name at all, not until her ending (in which she marries Takumi).
I'm SURE that at least a few users of Danbooru here have played the game Quilt, but unfortunately many of them don't seem to want to talk about it.
I looked at the official websites (both for the adult and all-ages version) and Japanese Wikipedia, but neither of them have any family names for any of the characters. Don't know anything about the game otherwise, so I couldn't confirm.