Infiltrating the Harem.

"In a far off land there lived a mighty Sorcerer King, cruel and all-powerful, he ruled his lands with an iron fist and crushed anyone who so much as dreamed of getting in his way. His forces were unstoppable, and many nations simply surrendered to him rather then face the decimation his armies would bring down upon them. For all of his malice however, the King had a weakness of sorts: He loved his many concubines, each a goddess in his eyes. He loved his wives dearly, and he took many, nearly doubling his collection each time he conquered a new land. These women were treated like living goddesses, nothing was denied them, the finest foods, clothing, jewels, servants, pets, it was said he would even pluck the stars from the sky to present them to his beloved wives. The problem was, he simply had so many he often forgot what some looked like, names would be a blur, but something that couldn't be forgotten was the large sapphire necklace that was presented to each one. Why, if a young lady somehow got her hands on one of these sacred jewels, it would be nothing to simply blend in with the crowd and gain the life of leisure afforded to these lucky women." 

So if you hadn't guessed, YC would be a girl who somehow got ahold of one of those Sapphire Necklaces and decides to infiltrate the harem. It can be for any number of reasons, maybe she's a poor girl looking to score an easy life, maybe she's an assassin looking to get close enough to try and kill him, or maybe she's some poor schmuck who got forced into it by one of the wives looking to escape, everything's up for discussion.