Haruka Momo 
The Engagement of Marielle Clarac (Manga) Volume 6 [EPUB ebook] 

Following her reunion with Simeon at the masquerade ball, Marielle discovers that his arrest was actually a pretense intended to draw out the real traitor! Members of the navy are at the top of the suspects list, but what about the ominous white-masked figure from the masquerade? Or, could the criminal be…someone very close to Simeon? When Marielle witnesses a kidnapping connected to this plot, she’s determined to give pursuit, so with the mysterious honey-handsome gentleman as her escort, she dons a disguise and infiltrates a seedy brothel. Can she find the victim before it’s too late? And, after all the treachery is revealed, do they deserve to be saved?