The "Top" or dominant in any power exchange relationship (Dom, Domme, Mistress, Master, Daddy, Mommy, Uncle, etc.)

Despite that in the outside world, "Guardian" generally refers to the caretaker of a Biokid, the priorities and purpose that such a care-giver should have with a child applies nicely to anyone given personal authority over another in a special relationship.

Like a "Guardian Angel", or a "legal guardian", the Guardian in such a safe, loving relationship should be oriented toward taking care of their Charge, protecting them (whether it be from injury during a pure D/s scene, or from fear as an Innerkid, or from overspending with an old-fashioned wife), and over-all being as responsible for this person as is appropriate for the relationship, living up to the trust it takes for someone to grant personal authority over one's self to another.

So, we find Guardian to be an expressive, apt, and lovely term to fluidly indicate any sort of authority figure in any sort of Backside of Love relationship.