A Daring Masquerade by Mary Balogh

Kate Mannering, a lady’s companion, is mistaken for the lady herself and kidnapped by a dashing masked highwayman. When she hears his story, however, before being released, she decides to help him recover his stolen birthright and soon gets herself embroiled in danger from smugglers and a would-be seducer. Meanwhile, an annoying, indolent guest at the home of her employer insults her at every opportunity but rescues her from every danger, even when she does not need or appreciate his help. She is alarmed to find herself falling quite inappropriately for both the highwayman and the guest.