@coffee_2512 from what i inferred, juda's mom was a patient at dohwa's hospital. his parents didn't do their job and neglected the patient (the part about doctors, not doing the surgery, etc.), which ended up in juda's mom's death. when they were kids, a group of bad people kidnapped juda thinking she was a child of dohwa's family, abusing her which gave her severe trauma. basically they wronged her in every way, not dohwa directly but his family.
the truth gets revealed and people bully juda even more, dohwa tries to protect her but she doesn't want his protection knowing he's only doing that out of guilt. she pushes him away and leaves him behind, so he commits suicide (i think? it was only shown that he was standing on the bars of the high walkway) in tails and returns in heads (this is why i was uncertain, it looks like he attempted suicide but if he did he would've lost awareness).
in short, their relationship goes to shit bc dohwa's parents indirectly murdered juda's mom.
this is just what i understood from what i saw, but i might be wrong lol