Bibi is the stereotypical obsessive girlfriend with toxic behaviour. However the key is that all of her actions this far have been well within accepted morality, and something about her makes me think she might stay there whilst something else says she'll take it too far and that's where we get this delicious tension from.
The other thing that's super interesting about Bibi is that she's still trying to understand herself and she's noticing how Amy being introduced in her thus far lonely world (not that she minded) is also changing Bibi.
Let's threaten and scare off this random girl around Peter! Wait, I had fun spending time with her.
Let's trade away this convenient friendship for a guaranteed date! Wait, I feel really upset without her around.
Let's go on a practice date even though she's nothing like Peter! Wait, she looks really pretty.
Let's tie her up like I've fantasized doing to Peter! Wait, I prefer her smiling. (I'd say she crossed the line here when it comes to morality, but Amy then proceeds to get distracted by a cute baby Bibi and immediately moves on from it lmao)
Playing with character tropes and challenging reader expectations is always fun. So far, Bibi has always lived her life a certain way and has certain expectations for others around her (doesn't care how she treats them and doesn't care how they treat her). She's pretty forceful and selfish, but it never really mattered before bc she doesn't have anything or anyone important to lose (the mistakes she made with her rabbit and dog were impossible to take back, so even if she learned from it, the damage was already done. It's unknown whether she has some kind of disorder or was just an innocent child who has now grown to be emotionally neglected).
Seeing her trying to navigate her relationship with Amy is super juicy, bc while Amy is a doormat with low self esteem and has high bullshit tolerance, she's also genuinely easygoing and accepting of Bibi. Bibi might be forceful with others and doesn't really care about how they feel or their consent (the usual yandere trope of one sided feelings), but we can also tell that she's starting to learn that beyond her own feelings, Amy being happy is also important to her and she wants to keep her that way. Ex, you like my bed? How about I give you the entire thing?