I don't like however how everyone here is just saying it's abusive and just bashing on the story though, it's just adding onto the stigma of mental health.
I don't know, maybe I'm just seeing things wrong as I have BPD myself (as well as autism) so from my viewpoint I don't see things as negatively. If anything I sympathize with Sayumi, although she definitely should have communicated things better to Jun.
Remember folks, in relationships like this, communication is key.
I agree that communication is key, but I also think that Sayumi and Jun's relationship is abusive. Even if Sayumi is suffering from undiagnosed BPD, it's not an excuse for shoving her partner to the ground and then physically hurting her. While I sympathize with Sayumi, she stepped way over the line into domestic violence, and this has not been addressed for the rest of the storyline.
That said, Jun is not entirely blameless here, either. Because she loves Sayumi so much, she has great difficulty with setting and enforcing boundaries with her. She lets Sayumi off the hook for all the possessiveness and toxic talk until things get physical -- and then she slaps her back, going from 0 to 100 all at once. Even in the end, it looks like she will just let things slide again, allowing the cycle to continue.