Hakushaku to Yousei Torikaerareta Princess

Fairies may kidnap a human child and replace it with their own offspring: a changeling. Now, it seems that such an incident took place in one of the Blue Knight Earl’s territories, so Lydia, Edgar’s fairy doctor and reluctant fiancée, sets off by herself to investigate. Meanwhile, the earl is visited by some old pirate friends whom he would have preferred to have stayed in his past. They tell him that their companion, Betty, has been kidnapped by someone claiming to be the Blue Knight Earl, but this is the first Edgar’s heard of it... (Source: J-Novel Club)