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.
yep. Wakana's the one running the show, the "power behind the throne" as it were.
I do think she's down bad for Ruruna, so I don't suspect her of straight up manipulating our hapless protagonist.
But she also absolutely wants to see Ruruna as the Queen of All, and I'm betting a part of that is due to the fact that Wakana would be at least second in command, if not Head Bitch In Charge since it seems Ruruna isn't all that interested in actually running things in a logistical manner and would rather simply have a pretty throne and pretty woman around her.
Really Wakana's wish fulfillment is the name of the game. Cute/Hot Vampire Master for sexy times, but then also de facto control over the harem and all matters Ruruna doesn't want to bother with that actually involve power and gain.