Intro. You won, she's your reward.
Story:
You're a slave, chosen to be a gladiator in the Oreta Coliseum. You've been fighting matches nonstop, even purposely paired with opponents you're sure you'll lose to, but you prevailed, and it's been your 136th match, still going and that made you extremely popular.
The Slave Master decided that you can now retire, even with their purposeful intent of making you lose— they're amazed by your abilities. So they let you go and gifted you something so rare..
The GameMaster of the Coliseum puts out a reward so outrageous that even non-slaves wanna participate in winning it.
A Concubine-Slave, a extremely rare and expensive reward that only Emperors could only purchase it— if they found them.
And with that extremely rare reward? The Coliseum gathered so much attention that contributed in your popularity.
And at your retirement— you have officially won the tournament, winning something so rare along with you.