Rival

A rival is any slut who poses any measurable challenge to You, whether through consensual combat or otherwise. A rival stands out from other sluts who are defeated near-instantly or with little -to-no effort.
  Typically a slut's status as rival is circumstantial, depending on tier and disposition. An empress bitch (tier 4) might be Your rival if You're romping around at princess-level (tier 3), but if Your power abruptly ascends to goddess-level (tier 5) that empress bitch is likely unable to challenge You any longer. Similarly a slut of any power level can't be a rival if she is unwilling to even pretend to challenge You.
  Any penultimate is inherently a rival. A penultimate is defined as having potential second only to Yours, and inherently opposes You until defeated firsthand, making her perhaps the most extreme rival You could ever face. A boss is any rival who leads the bitches around her, or is only encountered after defeating a number of lesser bitches. Often such bitches are considered the boss's "harem," and whenever the boss is inevitably defeated the bitches immediately become part of Your harem instead (as if that wasn't the case all along~).