Rival

  A rival is any slut who poses any challenge to You, through consensual combat or otherwise. She stands out from other sluts who are defeated near-instantly or with little effort (if any).
  A boss is any rival who in some capacity leads the bitches around her, or is only encountered after defeating a number of lesser bitches.
  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.