Hmm I feel like the last boss werewolf's going to be Izumi still, but maybe it's the president? She brought up that Yanagi had to be the werewolf that got Shion, but there was only one antidote stolen right? That should mean there's only one werewolf, but I don't know why Yanagi would cover for someone else.
If I didn't see the next volume cover my suspicions would be 100% on Ai but it looks like she's getting got next