Both shows have Shin Hye Sun though LOTBS was before SHS became a lead actress. Both shows are time travel fantasy romances where good eventually triumph over evil.
Both are dramas that involve a pair of lovers whose story ended tragically in history. Now they are reincarnated and reunited in present day, but one has a completely different personality.
Both are about the relationship between a human and a mermaid/merman who they end up living with.
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