Fair-haired boy and One and only

One and only and fair-haired boy are semantically related in sweetheart topic. In some cases you can use "One and only" instead a noun phrase "Fair-haired boy", when it comes to topics like darling.

One and only adjective - Eminent beyond or above comparison.

Fair-haired boy and one and only are semantically related in sweetheart topic. You can use "Fair-haired boy" instead a noun phrase "One and only", if it concerns topics such as darling.