I want this to go poly, but I just can't see how. Natori only has eyes on Lapis and only looks at Diana as the main character who will 'always' win in the end. Thus Diana has been rendered as a secondary character to Natori since she wants to save Lapis. And that's were it goes wrong, Diana sees and notices Natori. Diana has already changed, from the personal interventions and interactions, between the two Natori as remembers her from the game.
Still don't know what Diana what's from Natori, but if it is the same thing as Laspis a poly isn't likely to happen. If Natori does come around and finally 'see and notices' Diana there is going to be some serious indigestion between the two over Natori. And, by then, it may too late for a polyamory relationship. I could be wrong, hope I'm am, but I'm not seeing enough meaningful connection between Natori and Diana to warrant more than friends.