>>531020240
I've seen many theories as to what Millennium represents, but in general they seem to be a group of "modern gods", beings who represent scientific, mathematical and technological aspects of modern society that have become so interwoven into daily life that many treat them like we did the gods of old.

Which is why most of them aren't really "supernatural" in their essence, because none of them are anything supernatural in the first place, unless you count things like Koyuki's unpredictability or Asuna's super luck. The only students whose identities are basically confirmed are Koyuki (Chaos Theory) and Yuuka (The mathematical concept of Pi). If I had to hazard a guess at Momoi and the rest of the GDD's mystics it'd probably be either representations of specific game development studios themselves (given they're all named after famous eroge creators) or maybe more abstract concepts regarding gaming, like the concept of high-skill gaming (Yuzu), game art design (Midori), game story design (Momoi) and the general gaming fanbase (Alice). Who knows though.

This does put Rei and Sumire in a funny position though, since it implies they're representative of fitness nuts and baseball nerds respectively.