This is what i read from the previous chaptes and understood..idk if it's right tho.
Mir in the original was like a seed of jealousy and darkness and since aria appeared and got the care and concern she needed and how smiley aria always looked, mir got angered and jealous. Mir hated it and so she didn't like aria so when she found out her identity she left immediately leaving aria left to fend for herself but when they grew up, they met again and somehow while Mir recieved criticisms and bad talks behind her back while Aria became the queen of the social world and was still living a good life. Jealousy caused Mir became a villainess but because over here, now that our mc has become Mir..Aria instead was like the only light in her dark world the moment she saw Aria so she loves Aria (as to how she acted differently from the novel..well, idk but she herself is a person, has her own personality and soul and i dont think the book's Mir personality has to become our mc's personality..basically our mc is just a good person shshshsjd)