Wait, I literally just realized that Mai’s real name is Miyuki.
I am… not sure that’s the kind of nickname Japanese people would use? Like, it’s just a completely different name. It doesn’t even sound similar, besides the initial M. (It’s like if a an American guy named “Jonathan” had the nickname “Scott” or something.)
I think a more typical nickname would be Mii-chan or maybe Miyu-chan—though I’m less convinced, because just “Miyu” is also a different, real name—but even then, I believe that in Japanese culture nicknames are almost exclusively for family and childhood friends; you have to be pretty damn close to someone for them to even let you call them by their given name, instead of their surname or a title. I can get why a game like this would simplify things a bit and just refer to everyone by their given name, but using nicknames American-style like this does go against that “glorious Nippon steel” vibe Tobs is going for LOL.
Side-note: the Japanese had a culture of name-swapping—that is to say, changing one’s name upon hitting certain milestones in life (becoming an adult, starting a new career, receiving a title, etc.)—but in that case Miyuki would just go by “Mai”—she wouldn’t do any of that American “My name is John Fergusson, but you can call me Scotty *wink*.” (Also, as far as I know, people who did this generally preserved one element of their old name in their new one; so upon becoming a priestess, for instance, Miyuki might swap her name to “Misora”, or “Yukiko”; but “Mai” is kind of random.