Not much is known about the relationship between Stumpy and Eugly, as they barely interact on the show.
Stumpy doesn't talk much with Eugly, but he occasionally does talk about her. Usually, this involves insulting her in some way, for example in "Let's Play Tutus Galore" when he, along with everyone else, laughs at the prospect of Eugly being a ballerina.
Eugly sometimes beats Stumpy up on Pretty's orders. However, she rarely does this of her own accord. In "Let's Play Castaways", she decides to rescue him and the rest of the main four from an island. She manages to find him and Kaeloo on one side of the island and carries them to safety, ignoring the fact that Stumpy is chewing on her since he is hungry.