In "The Handmaid's Tale", what is Nick's role in Offred's relationship with the Commander?

Nick, the commander's chauffeur, functions as a messenger throughout the her series of clandestine meetings with the Commander. When he wears his hat sideways, she knows that she is to go see the Commander that night. Later, the Commander's wife arranges for the narrator to get to Nick's room safely at night in order to become pregnant by him, since it appears that the Commander is sterile. She keeps her affair with Nick going, sneaking to his room over the garage even without the approval of the Commander's wife.