I cannot fins anything about them in thepeerage.
These are The National Gallery's e-notes for this work: "'The Morning Walk' by Thomas Gainsborough shows an elegant young couple strolling through a woodland landscape, an attentive dog at the lady's heel. William Hallett and Elizabeth Stephen were both aged 21 and due to be married in the summer of 1785, shortly after the painting was completed.
Portraits of wealthy sitters posed in a natural setting and dressed in their finest (but not necessarily most practical) clothes were a popular status symbol.
William is in a black, silk velvet frock-suit. His apparent carelessness is actually a studied pose. The undone jacket and with one hand tucked into it is a stance seen in many fashionable 18th-centry informal portraits (known as conversation pieces). 'John Plampin,' also by Gainsborough does the same.
Elizabeth is in a dress of ivory silk - perhaps her wedding dress - caught at the waist with a black silk band. A frilled muslin kerchief covers her breast, with a knot of grape-green ribbon under it.
The light, feathery brushstrokes used to describe the landscape are typical of Gainsborough's late style. William's hair and Elizabeth's gauzy shawl almost blend into the landscape they walk through."