When we first meet Denis, he is a wealthy man, the owner of a grocery store, and also the person who gives the narrator rides to the train. When Denis's horses all die, the narrator is forced to hire Ethan. So, Denis provides a sort of sideways connection to Ethan.
It turns out that Denis lived in Starkfield 24 years ago, was interested in Mattie Silver, and probably wanted to marry her. Young Ethan was jealous of him, but Mattie never seems to have had any interest in Denis whatsoever.