The Marquis who Mustn't - Courtney Milan - Bog

One good fraud deserves another...
Miss Naomi Kwan has long wanted to take ambulance classes so that she can save lives. But when she tries to register, she's told she needs permission from the man in charge of her. It would be incredibly wrong to claim that the tall, taciturn Chinese nobleman she just met is her fiancé, but Naomi is desperate, and desperate times call for fake engagements. To her unending surprise, Liu Ji Kai goes along with her ruse.
It's not that Kai is nice. He's in Wedgeford to practice his family business, and there's no room for "nice" when you're out to steal a fortune. It's not that the engagement is convenient; a fake fiancée winding herself into his life and his heart is suboptimal when he plans to commit fraud and flee the country.
His reason is simple: Kai and Naomi were betrothed as children. He may have disappeared for twenty years, but their engagement isn't actually fake. It's the only truth he's telling.