It's a bit scary how darn *accurately* this webtoon depicts teen/early adult girl's thoughts and behaviors... On another note: Loony girl focusing on Sera so intensely and from so early on makes me suspect she had made some kind of sketchy deal with that shady old man of the creepy eyes to do witchcraft on Sera and curse her.
Spoiler about their relationship
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The crazy bitch was born cursed, she would never be loved, never happy. Her parents found the shaman and he said that the only way to "cure" Crazie from the curse, was passing it to another child.
Crazie them decides she wish to pass on to Sera. Because of that, Sera's started acting like a crazy mad woman, she was conscious about her acts and that she was hurting people, but she was unable to stop.
So, the red haired "doctor" is a villain sent in by Erusha's estranged maternal family... Either her maternal family are bad guys, or they're being manipulated too.