beta
So from my understanding, Seo-In has an obsessive personality type but is afraid to reach out first because as a child, he's felt that rejection of love from his adoptive parents. As a result, he tends to probe and prod at others to essentially test and push their affection. We saw that with the couple as children, and it also explains why he's so push and pull with Yeongwoon. He tends to be sweet when the MC is, but retaliates at any slight hint of Yeongwoon pulling away.
Yeonwoon is also seen to be a little possessive of the ML(although within very reasonable bounds) but because he feels that he's been played with and used by Seo-In in the past as a stand-in(at least in his mind), now he's particularly wary of getting too emotionally invested. So he tends to push and pull too, but bc he's got this whole inner conflict going on.
Jaewon is my least fav just bc he feels very one-dimensional. He repeats the same lines over and over again, 'youre my shadow'. If we at least saw signs of him being upset by their parents attention to Seo-In, or that he was greedy for his family's attention, id understand him more. I wonder if the author will flesh out more about how his illness impacted his mental health bc most times we see him, the dude seems fine. But someone once said he's not a real antagonist, but rather a plot device, which i think makes his repetitive character traits more bearable.
The mom is interesting. She definitely cares for Seo-In, but I think she just prioritizes the idea of her perfect, unbroken family more. She might actually be more biased towards Seo-In as the stand-in for her dead son and the one that followed in her footsteps than Jaewon.