I like this theory. Maybe Sejin just went through so much trauma that he lost his memory since the guy in the bathroom was like "you don't remember me?" But that would mean the brother was seeing Sejin in secret and only Ido knew because the rest of the family didn't seem to know Sejin at the engagement ceremony. My initial theory was that this is another life. In the first life Sejin and Ido crossed path, and Ido couldn't do anything to help Sejin when he was being abused. ╮(￣▽￣)╭
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I put the italian hand emoji after "this comment is" showing that i like Yoongles's comment (in case the emoji didnt show up)
I think this is more of reincarnation plot rather than amnesia one. If Sejin has amnesia, their family would act differently. Tho, it's not impossible since I think Sejin's assistant, Minjae, Ido, and Ido's bro know something.
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The brother raped sejin and ido did too afterwards. Ido loved sejin but after he was raped by the brother he assumed what happened was consensual. After this he became obsessive and hateful to him because he thought sejin didnt actually love him then he raped him. when he learned the truth it was too late sejin killed himself for everything they did to him. Its really tragic
I might be crazy fro saying this but I think it's Ido's bro that was in the past treating Sejin like a shit and Ido was watching on the side (I mean, he witnessed and can't do anything). Why? First, maybe it's just me but when I saw Ido's bro, I thought it was the angry Ido. They're really alike, appearance wise. I can't really tell them apart until when I saw them together (when their family is entering) BUT ITS WAS SUBTLE DIFFERENCE. Second, Ido's been dropping clues that he knew the past but there's was no sign of him doing repentance actions. Like, when he said 'Ive been longing for this for a long time' (non verbatim) when they first met, it was as if there's no way he treated Sejin like shit. Like, he's waiting for this moment to treat Sejin better because he's always been mistreated. AND, if it was Ido who's the shitty guy in the past, HOW WOULD HE KNOW WHAT SEJIN LIKES AND WANTS? If he's mistreating Sejin, how can Sejin brought him to Han river? Thought him french? The bathroom style? The garden? The preferences? I mean, if Ido hated Sejin, why would he spent time listening to Sejin? And the flashbacks on Sejin dreams doesn't match up with Ido. In Sejin's he's being mistreated but in reality they're doing fluff things and Ido predicted things as if it was aligned with the past. If it was Ido, HOW CAN HE MISTREAT SEJIN WHILE BEING GOOD TO HIM?!.
Okay, it's time for me to touch some grass