Hakushaku to yousei, book 3: Gentle with thy Proposal
Mizue Tani2005

Lydia is a girl who can see fairies and is hired as an advising Fairy Doctor to the Earl Lord Edgar, a rake with a complicated past. One day, a field flower fairy comes to pay them a visit. The fairy came to escort Edgar, the Blue Knight Earl back as the groom for her fairy Queen, but says that the 'moon' she was suppose to give to the groom was stolen. At the same time, a ferocious kelpie arrives in London from Lydia's hometown. Apparantely Lydia had once been proposed by him...!?