Sung Yohan and Jung Bareum swapping places throughout their lives. Beyond the obvious switch, we know that Yohan as a child was happy and kind. Jaehun was envious of that, the way that Yohan could get everything "right" where Jaehun was criticised for reasons he didn't fully comprehend. When Bareum reinvented himself after his family's death, Yohan was clearly the template.
But in the meantime, Yohan was subjected to the full force of society believing him to be the same as Han Seojun. And whilst he could weather it to an extent, eventually it did take his toll, turning him from that cheerful child to a man willing to take his own life just to escape. But he was convinced to live, to become a doctor and do what he could to help, regardless of how he was seen by the outside world. To try to uncover the experiment that was his life and make a bid to escape it.
Bareum at the same time found revenge for his family and dove deeper into the trap, unaware of it closing around him. He chose his victims, and unwittingly left behind a trail for Yohan to follow back to Oz.
And when Yohan did die, he left behind an amnesiac Bareum, who only knew himself as the mimicry of the younger Yohan he'd become over the years. Who knew nothing of mice and traps, but who followed in Yohan's footsteps as he discovered the experiments for himself.
Bareum unknowingly stealing Yohan's life yet again, until Yohan steals it right back from beyond the grave, cursing him with the emotions he'd had to endure in his place.
