The string that faded with the girl and yeon-ah feels like she's the original destined for our cutie top. Remember when Yeon had this dream (Joseon Era, I think) - when he suddenly collapse after reading on how to forcedly connect your destined one - that dream when the young Yeon tied the string of fate on our sunny bottom, looks like our Joseon Era Yeon died (base on Kihoon's dream in their past life) as the consequence of forcing their string to be connected. And their life on the present is a reincarnation for them to be together forever or for them to disconnect the forced destiny.
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The string that faded with the girl and yeon-ah feels like she's the original destined for our cutie top. Remember when Yeon had this dream (Joseon Era, I think) - when he suddenly collapse after reading on how to forcedly connect your destined one - that dream when the young Yeon tied the string of fate on our sunny bottom, looks like our Joseon Era Yeon died (base on Kihoon's dream in their past life) as the consequence of forcing their string to be connected. And their life on the present is a reincarnation for them to be together forever or for them to disconnect the forced destiny.