It’s hinted at in the beginning and confirmed in the dream flash back that gives Yeongwoon a panic attack. Where the kid that looks exactly like Seo-in says he’s late because he had to say goodbye to his brother at the airport. This aligns with the opposite flashback Seo-in had of waiting for Yeongwoon with his mother saying she didn’t think he was coming.
I know! I was confused at the beginning and with this theory I’m more confused
was he not the step brother who died??
Ya'll, I don't know where everyone is getting the twin shit from. It's frying my brain