(other than originally being bought by the scientists to try to breed with Saya consensually opposed to her poor sister, Diva, which is hinted at being forcedly raped by an unknown suitor, imprisoned inside her tower. long explanation. they were a new species, and these two scientists wanted to study them. Diva was locked up in the tower to keep human influence to a minimum, to study her "naturally/wildly", while Saya was allowed freedom to live with humans and be influenced by them, studying her "unnaturally/tamedly/domesticatedly"), as the human scientists wanted to impregnant them, being such a rare new species and dangerously consisting of only themselves. They didn't know at the time how this new species, chiropterans, reproduced).
longer story:
A new species is found, literally a batwoman, who they call Saya (the mother), which they call chiropteran. chiropteran is the actual scientific name for bats, btw. they are NOT vampires, but literally batmen and batwomen. Mother Saya is dead, and these two rich scientists begin to dissect her, finding two egg-cacoons in her womb. Their scalpels couldn't penetrate the hard egg-cacoons, and accidentally Amshel's finger gets cut from the sliding scalpel, and a drop of blood falls unto one of the egg-cacoons. It begins to pulse-glow, and it starts to hatch, the other egg-cacoon is blood-dripped and it hatches too. They name the two seemingly human baby girls, Saya and Diva. As scientists they want to study them. As you know there's a difference from a wild animal uninfluenced by humans and a domesticated tamed animal influenced by humans. so, they decide to as a control to test both. Diva would be locked up in a tower to keep human contact to a minimum. While Saya would be allowed to live among humans. They would compare the results and differences. Amshel was in charge of Diva, and Joel was in charge of Saya. Part of their study was these chiropterans reproduction, being concerned that there were only two of them, Saya and Diva. It was hinted at poor Diva being forcedly raped by an unknown suitor, while Saya would be bought a slave boy, Haji, in hopes that with time, she'd come to love him and thus have consensual sex. Well, this is not how their reproduction worked. These chiropterans, Saya and Diva, were the Queens, of their hive like species. the eventual truth of their species is that: they bite humans, making male drone-suitor chiropterans known as chevalier. Amshel became Diva's first chevalier. Haji would become Saya's first chevalier due to him falling to his death to get a flower for Saya down a ways of a cliff wall, lol. She had seen how her own blood would heal herself when she got hurt, so she tried to give her blood to the dead haji on the ground below. he's brought back from the dead to become her first chavalier, instead. anyways, for Saya or Diva to get impregnated, it turns out that it can ONLY happen from a chevalier of the OTHER Queen. Amshel figures this out, and so Diva rapes Saya's 2nd and last chevalier, Riku, her foster little human brother, and then kills him after raping him. Diva gets pregant, and unique to their species, they always and can only have two babies, two girl babies. Saya would eventually kill Diva, but spares Diva's babies.
the plot is a war between Diva and Saya. Diva wants to help her own species, turning all of humanity into chiropterans with her chavlier Amshel's (along with her other chevaliers') help, and Saya wants to protect her adopted species, the humans, fighting for them against Diva, with ehr faithful Haji always at her side.