Tyler Durden interacted directly with other people.
Fair enough, and while it's a different situation than the one I was describing, the careful crafting of the author still applies. Tyler was an alter ego, who was never spoken to or acknowledged by other characters at the same time as the narrator. There's several shots of Nikaidou and Tadokoro together with people talking about both of them by name (including Tadokoro's own mother), so according to the evidence, she's a real person. If there's some reveal later that she wasn't, then it wouldn't be supported by the evidence and would just be lazy storytelling.
Still, just in case we did meet Tadokoro at a very strange time in her life, I'm keeping this on stand by (I really like Fight Club).