You're not alone. From the start, I really thought she was the leading lady. She was just whorish at first, but she is genuine. But I think it is safe to assume that double body girl is Mc's partner. Unless it turns out she is a relative.
I personally believe that they are siblings. She has a dad but no mom, he has a mom but no dad, and their OG bodies look so alike that they're mistaken as siblings, both being short, chubby, and lacking eyesight. In fact, I like to think of this body splitting as something hereditary, inherited through the dad. He could be a semi useless parson married to MC's mother, but his OG body died and such and such. That would explain how he knows about both's body splitting. She claims he's the only one, who knows about her, while he clearly also knows something about MC else he wouldn't have sent his daughter for just a prospective talent.
So, I'm probably the only one, but I seriously like Ha Neul/Hyung Suk. Like, I ship it?? She was pretty terrible at first, but she's grown a lot and as of now she's the only character that likes Hyung Suk for all that he is, real body and magic body. They seem like they could be a really good couple.
Don't get me wrong, if I thought for even a second Hyung Suk might be gay, I'd be alllll over JaeSuk (HyungYeol??). Alas, I can only wish for Jae Yeol to find a wonderful guy who returns his affections. Find love, JaeYeol~ ヾ(❀╹◡╹)ノ~.