Character Analysis
Mr. and Mrs. Underwood aren't big characters and Summer only meets them once, however they help Summer understand Cletus's place in the world and how she's been too harsh on him all along:
Now, meeting these sweet people, I knew right away it wasn't them Cletus was ashamed of. It was me. Ashamed of me and my indifference to him, afraid to let his parents see the way I barely tolerated their strange son. Ashamed of the difference between their adoration of him and my disgust. (8.22)
Seeing how sweet and kind his parents are makes Summer realize that Cletus is a good person too—he's always there to check on Uncle Ob and is truly concerned for his health and well-being, after all. When Summer sees Cletus fit into his family right between his parents, she sees him in a different light.