How Can I Convince My Players That This Evil Character Is Good For the Duration Of The Campaign Until The Big Reveal?

My game has a woman who is a noblewoman. Despite her low-status birth and life as a peasant, she would later be seen as a miracle child because of in-world lore related to a physical feature of hers. This allowed her to get many followers who helped push her into the ranks of lower-“nobility” as a baronetess. From there she manipulated her followers in order to make her actions seem more praise-worthy and honorable that she eventually was made a trusted adviser of the king’s, granting her a formal nobility rank as a Countess and, as a result, allowing her to run her criminal operations more smoothly.

Okay, lore aside… She is an absolutely beautiful and cunning woman. She never lies because she doesn’t ever have to thanks to her wording. She has a high charisma and wisdom, so if someone were to question her, she would always be multiple steps ahead of them so that she doesn’t have to say anything that is false. That said, one of her quirks is that she tries to collect (Read: “Enslave”/”Own”) anything that she finds to be unique or valuable and she sets her eyes on one of the PCs.

The PCs just shut down a large part of the kingdom’s slave trade. Slavery isn’t illegal, but it is something the king is trying to edge out of their society. At the same time, he can’t request people to shut down and deincentivise slavers lest he causes unrest in his nobility. He trusts Countess who he does not realize 1) owns many slaves that she keeps hidden in secret and 2) is the one in charge of the vast majority of the kingdom’s slave trade; therefore, the king has invited Countess to the celebration of the PCs shutting down such a large aspect, freeing many slaves and getting them someplace safe. Publicly, the celebration is under the claim that the PCs found a lost artifact (which they do by chance) belonging to the royal family (and therefore the kingdom) by right and it’s to honor their (the PCs’) service to the crown. Everybody who is invited to this celebration are those whom the king trusts with the information relating to the true reason for the party, guests and staff alike.

At the party, Countess will try to seduce one of the PCs by putting a “love potion” of sorts into a drink which she will then offer her target. (I as the DM expect the player to succeed the DC to be unaffected by the immediate effects of the potion, so mentioning the potion is more “side information” than anything.) That said, when they are talking, I need to know how specifically to keep her from coming across as obviously suspicious since she will be directly approaching one of my players and will have a noticeable impact on him thanks to a very “Strahd von Zorovich” style Charm skill while they are talking, regardless of if he drinks from the spiked glass or not.

The problem: My players are quite genre-savvy. When I do this, I expect them to figure out that she is suspicious, if not evil, but I want to avoid her being seen as even suspicious. What is the best way to do this while maintaining the story as intended? This is done through a homebrew system, but if you must, assume D&D’s system. If you need my party’s (the PCs, not the celebration,) information in order to answer this accurately, let me know.

Question: How can I convince my players, not just the characters, that this evil character is actually good for a prolonged length of time until the final reveal?

A best answer will be almost entirely through minimal use of rolls and maximized use of story-telling. Rolling would just add to the suspicion and I don’t want my players to even think they are on to me.

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