I love the game and I'm dying for an update (FYI it's a crime calypso isn't one of the main girls)
I think in part it may be because the game is initially presenting Eternum as simply a virtual world, and the main paths are meant only for "real" people, i.e. people you meet in the real world. It'd be an interesting thing if she suddenly counted as a major/main character halfway through the game though, after (for instance) a discovery that Eternum is more than just a game. A change in format going along with a change in the story, so to speak.