This is spoiler but
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Basically, Yohan is crazy. He always acts like he is being alienated but it is his own actions that alienated him. We're getting closer to it but Jun will have the chance to talk to Yohan's sister and she'll tell him that Yohan has an inferiority complex because he has a lower iq than her. Yohan isolated himself (not his family) and even dramatized his family life to gain sympathy especially towards Jun. When Jun first came to their house, Yohan hid photos of himself so it'd look like he was being neglected/excluded
More spoilers
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An ironic thing is that apparently, Yohan might have actually been abused by his father, which the other siblings aren't aware of. He was once hit so hard in the head that he lost all his memories, and after that is when he started behaving crazy. The official story in the family though (and what Yohan himself believes), is that he fell down the stairs and hit his head bad causing the memory loss. So he is a victim, but he doesn't remember it and even fakes being another sort of victim lol

why do yohan's parents hate him so much?