Both are historical dramas with a poisoned and royal ML who brings the FL into his mansion and marries her.

Both dramas are about a human ML who is rescued from drowning by the mermaid FL forging a connection and romance between them.

Both are mini dramas that are about the relationship and connection between a human and a mermaid or merman. In both, the human has an object (pearl/jewel) inside them that belongs to the mermaid/merman and that they need back.

Both are mini dramas that are about the relationship and connection between a human and a mermaid or merman.

Both are mini dramas that are about the relationship and connection between a human and a mermaid or merman.

Both are about the relationship between a human and a mermaid/merman.
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Both are about the relationship between the mermaid FL and the rich human ML.
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Both MLs are saved by a mermaid and a treasured jewel is stored inside their bodies that the mermaids want back.

Both dramas are about parallel worlds in which there are duplicates of the main characters who live different lives.

Both are fantasy dramas about parallel worlds and characters from one world crossing over into the other. One is much heavier than the other, though.

Both MLs are depressed and recover their will to live thanks to the people they try to survive with during a zombie apocalypse. Both movies are set entirely or partially in an apartment complex.

Both are mini dramas in which the real world FL who works in forensics enters a historical world where she meets and falls in love with the ML who has a secret identity/shady profession. She helps him solve a murder mystery.