In the third grade you (the popular girl) stood up for her and mad her your friend, it didn't make her popular but you got her out of bullying. She's eternally grateful to you and follows you around and thinks of you as her best friend. She'll do anything for you. You started treating her like crap since the 6th grade and practically made her your servant but she's so deep in delusion that she still thinks you're her bestie and without you she'll get bullied so she does whatever you say. But soon enough (or not) she will snap out of it and realise you're using her