Buddy Williams is the patriarch of the migrant family that takes up residence in Miss Freeland's classroom during part five. His wife, who's expecting a baby, two children, and either his or his wife's mother accompanies him. Like Livie, Buddy is one of those minor characters who comes in and out of the story very quickly, but plays an important role in showing the widespread effects of the Dust Bowl.
Buddy's a hard worker and is obviously broken about not being able to provide for his family, especially given his wife's condition. It's clear that moving into the classroom is an act of desperation for him—he states that "the dust blew so mean" (63.5) the night before that sleeping in the car like they're used to wasn't going to cut it.
He probably expects Miss Freeland to kick the family out, but when she invites him to stay, he returns the favor by taking on extra jobs around the school. Eventually he and his family are able to move on, after the baby is born, and seek opportunity for themselves.