The only other girl that will eventually get her own status screen is A, and this is for a reason:
The main cast includes Olivia, Allison, Maddison, Brooke, and A, and only these 5 have their own "Pure, Dark, Lust" variables.
Side characters still have "paths" and various relationship types, but they're more straightforward and moreso related to direct choices rather than any points system.
Radiant is a harem game that, unlike many harem games, allows you to actually decide who you're in a relationship with.
So to answer your question: yeah, you *can* exclusively romance and end the game with Mira if you prefer that, but since the game is mainly intended to be played with a *full harem*, updates will be shorter for you and there won't be as much content as there would be if you were instead romancing someone from the main cast.
Side girls are romanceable and you can remain in a relationship with them even up until the very end of the game:
Their "paths", and subsequently your choices, just function differently than they do with the main girls, as side characters don't have full-fledged points systems.
