The harem lead is not Ruruna. It's quite obviously Aihara Wakana and she's just using Ruruna as a convenient tool to expand her own conquest. Her mom is the president of the school board, right? Almost like she thinks the place was built to bring her women to conquer. It was Wakana's idea that she should bite everyone in the school and just take it over. She even asked Ruruna to fully turn her so she could make her own kin a while back. Ruruna decided with Wakana's ambition that might destroy the world if she did that so she declined. Probably a smart move.