Title: The Crown's Shadow
A random boy from Korea finds himself in another world. While traversing this foreign land, he comes face-to-face with someone who looks and sounds identical to him, like his otherworldly doppelganger. It turns out that this person is Daniel, prince of Rowen. Rowen makes the other-worlder an offer: I'll take care of you if you become my shadow.
Having nowhere else to go, the boy accepts and is given the name Rael. Rael and Daniel trade places constantly, playing the prince's role and sharing the burden all of that entails. This goes on for years, with no one catching on, including Daniel's closest family. Things start to go awry, though, when the kingdom's emperor comes for a visit.
The emperor is immediately entranced by Rael, who is playing Daniel, and becomes obsessed with him. When Rael and Daniel switch back, the emperor notices immediately but only becomes more intrigued. Finally, on the day the emperor is set to leave Rowen, he goes to Daniel's father and asks him for a single night with his son, and in exchange, he will give him trade routes and mines. It is an offer the kingdom of Rowen cannot refuse. Daniel, of course, does not want to do this, so Rael, thankful for all that Daniel has done for him, offers to go in his stead.
Rael goes and ends up spending three nights with the emperor before they part ways, and even Rael has to admit that he fell for the emperor, but he admits they may never meet again and moves on. However, he is thrust back into the emperor's lap when Rowen is suddenly overcome by a noble uprising, and Daniel is murdered. Forced to take the role of Daniel permanently, Rael rushes to the only person he thinks can help him: emperor Lionhart.
Unfortunately, Rael doesn't know that while the emperor is happy to open his doors to Rael, he won't be so keen just to let him leave again.