SPOILERS
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Alright everyone, so the truth is, the one who got kidnapped is actually Yung Jun! And what happened was that, when he was a kid, he went to that area with his brother, who left him there. A crazy woman found him and kidnapped him and Kim Miso. After they escaped and went back home, Lee Sung Yeon (the big bro) went to see the place where Yung Jun was locked in and he got really shocked and felt so so guilty that he got a psychological problem. The big bro is actually mentally ill, he started seriously thinking that he was the one who was kidnapped. At that time the whole family was in a chaos, nothing was like before, everyone was suffering, and when Yung Jun saw that he pretended that he has lost him memory. So actually he rememberes everything. And the parents didn't want their kids to suffer so they didn't tell the truth to Sung Yeon.
At first i also thought that Sungyeon was the one who actually left Yungjoon but then I was like, "No way a kid would do that, that's so sinister". Turns out my gut feeling was right... I almost got it right about the parents pretending thing, but the one they're putting the act on for is the big bro instead...
that since Yungjoon had amnesia about the kidnapping his parents pretended that his older bro was the only one who was kidnapped (and they didn't tell Yungjoon he was kidnapped so as not to agitate his amnesia situation)? and then older bro just embellished the story he told Miso about it being Yungjoon's fault?