Plot and main leads are very similar, except that in Devil Beside You the two leads are soon-to-be stepsiblings.
Main character had a crush on her childhood friend for the longest time. However, there was a girl he got involved with that he couldn't reject. Leading male's parent also abandoned him at a young age, but meet again years later.
In both, the main character had traveled through time and was forced to act as the lookalike imposter of the king from that period. He attempts to unify the kingdoms into one country.