the reason Jaehyun is "obsessed" with Jeongmin is Jeongmin is the only friend Jaehyun has felt actually understood her (aka felt like a friend). That's why she stopped seeing her "deliquent" friends at Jeongmin's request. In middle school, Jaehyun (as a coping mechanism) developed a manipulative persona and while she has followers/minions, she doesn't have an actual friend she can trust or feel a sense of kinship like she did with Jeongmin