Mona is trying to get Kurumi to like her in order to use her to manipulate Naoi, right? I don't think she has outright malice or even necessarily knows what she's doing so much as she lives in an emotionally unstable and manipulative environment, with the baby being used to paper over the issues in their mom and stepdad's relationship.
But her empty eyes in the last panel makes me feel like it could get nasty. Kurumi also seems more open to her than most other people due to the resemblance to Naoi, she's even been very slightly blushing (cute!), and she's emotionally vulnerable due to reaching out to her mom...
