When dealing with the fates of gods and goddesses, or more importantly, omnipotent beings who know every damn thing before it has the faintest glimmer of occuring, it's pretty feasible that Mara's partner in the doublet system, if she is to redeem and turn to the side of good, would have been a demon at the outset.
Of course, it could have started as, and remain as an angel, for the sake of creating a nifty plot that most people wouldn't pause in considering.
Of course, it could have started as, and remain as an angel, for the sake of creating a nifty plot that most people wouldn't pause in considering.