Along with Livie's alleged appearance in the slats of the boxcar, the drifter from Moline, Kansas who joins Billie Jo in her journey plays a huge role in getting her to reverse her decision to leave home.
When he first gets into the car, it's clear this guy's been through the ringer—he stinks, is covered with train soot and dirt, and looks completely worn down by life. We learn as he talks to Billie Jo that he, like Daddy, is a struggling farmer. He feels overwhelming guilt for not being able to provide for his family, and has left home in hopes of being able to find a job to support them.
There's some stuff about this guy, though, that doesn't quite add up. When he leaves the boxcar as Billie Jo sleeps, he steals her biscuits and leaves behind the picture of his sons and wife, his only possession. For someone who just pages ago was eaten up by shame for "taking food from a child" (100.2), this seems kind of contradictory.
Before we completely write him off for the Dust Bowl equivalent of stealing candy from a baby, let's note that it's possible there's greater meaning to this seemingly selfish gesture. Because seeing the photo of his family seems to be the last straw in moving Billie Jo to go back home, could it be that the drifter left it as encouragement for her to do just that? We can't say for sure, but we do know that the man plays a pivotal role in the climax of the story.