part of me thinks there must be another big twist besides Ruri being the bad guy.
Perhaps one of the other idols also being involved in Arika's disappearance, or perhaps by Arika's matter they're talking about how they can rescue Arika. Although that wouldn't explain why they had arika's clothes and didn't go to the police though.
Maybe the simplest explanation is the best but then I'm not sure if there's enough mystery and story to go off if they confirm the bad guy this early