I did have my suspicions when that girl tried to woo the Demon King, but based on everything else, the way things are she could've just been double faced manipulating bitch without necessarily being a reincarnator, and that's how I took it since there never was any 'reveal' of her being a reincarnator. Then there's also how she reacted to the Demon King's appearance the first time she saw him(when he helped out the female lead who had saved the fenrir pup), which seemingly later made her try to woo the Demon King. That's how I interpreted it. There's also her inherent and strong racism against the demons, which seemed to be like that of the nobles(for the most part).