The Wicked Heir by Elizabeth Michels

When the love of Lady Isabelle Fairlyn’s life is betrothed to her twin sister, Isabelle vows to find a suitable replacement before the end of the season. He must be a talented dancer, have a keen fashion sense, and be perfectly dashing in every way.

Fallon St. James is the farthest thing from perfectly anything. As head of the secretive Spare Heirs Society, he must stick to the shadows…even as Isabelle’s friendship pulls him reluctantly into the light. But when Isabelle gets involved with the one man who could destroy Spares, Fallon must decide between protecting his life’s work―or risking everything to save the woman whose warm smile leaves him breathless.