Geurimja Wanggwan

After waking up to find himself in another world, he comes face to face with the prince of the kingdom, Daniel, who looks exactly like him. In return for saving his life, the prince asks him to play his stand-in, basically to become his 'shadow.' He accepts and with a new name and identity, Rael serves the prince by taking on any given tasks. But what will he do when he's asked to spend a night with the emperor of the realm...?

(Source: Tappytoon)