Alright, so I had to go back and double-check this because it's been a minute since I last flipped through 'Revenge Wears a Mask'. I'm pretty sure the main antagonist is Jonathan Clarke, the protagonist's former business partner. The whole plot is driven by his betrayal—he basically stole the company and framed the main character for fraud, which kicks off the entire revenge journey.
What makes Clarke work so well, in my opinion, is that he's not a cartoon villain. He's a believable, coldly ambitious guy from the protagonist's own inner circle, which makes the betrayal cut deeper. The real tension comes from the fact that the heroine has to navigate high society events and business deals where he's always present, smiling and pretending nothing happened. It's that constant, simmering presence that really defines the antagonistic force.
There's also a secondary character, Victoria Sterling, who acts as a social antagonist, constantly trying to undermine the protagonist's reputation. But she feels more like a pawn or a symptom of the world Clarke manipulated. The core conflict always loops back to him and the systemic ruin he caused. The book does a solid job of making you hate him without him ever needing to twirl a mustache.