Just because you haven't watched "Merlin" doesn't mean you don't know the legend of Merlin, nor does it mean you don't know that King Arthur doesn't have a twin sister at all!
Well, it's really hard to hide your magic power from King Uther...
Also, as for the BG of this article, as for the male protagonist, I have not decided, maybe there is, maybe not, and Merlin and Arthur have a hard time choosing... Although the heroine and Arthur are twins, and I really don't want to break up the pair of Merlin and Arthur... But for the sake of the heroine... Let's break it up...