Wait what’s the story between the two exes again? I kinda forgot/maybe never really understodd to begin with?
Kaoru (Honoka's vamp sister) and Shizuka (Mio's girlfriend) used to date back in middle school but broke up because Kaoru kept fooling around with other girls. Shizuka initially wanted to cut all ties with her, but relented and agreed to stay friends after Kaoru stopped eating and going to school. They then entered the same high school and were officially just friends, but clearly remained romantically and possibly sexually involved.
When Kaoru discovered that Shizuka was growing closer to her lit club kouhai Mio, she initially sought to sabotage their relationship. However, she soon became infatuated with Mask and decided to drop Shizuka, which she, it turns out, did not want at all. Her attempt to get Kaoru back was a disaster that almost cost her Mio as well. They luckily got over it and cutting class to get Mio'd all day seems to have cured her of lingering feelings for Kaoru, who's still chasing Mask.