Hakushaku to Yousei

Lydia Carlton is a fairy doctor who can see and communicate with fairies. When she embarks on a journey overseas to visit her father in London, she's kidnapped by a young man named Edgar Ashenbert who introduces himself as an earl and requests Lydia's aid in searching for a treasured sword that will prove his status. Although she finds his story dubious, Lydia agrees to work with him—if only because he's her first real client. Meanwhile, rumors of a heinous burglary abound, and the culprit seems to bear an uncanny resemblance to Edgar...

(Source: J-Novel Club)