Mr. Hardly is the shopkeeper of the town general store. While he runs a good establishment, some of his business is a little bit funny, as he charges too much for substandard goods and often cheats his customers and shorts them on change when handling money.
Although Billie Jo describes him as not too nice of a guy, there's room for ambiguity in his character. It is unclear whether he accidentally gives Billie Jo more change than needed when she goes to buy the ingredients for Daddy's birthday cake, or if he does so on purpose, knowing her family's situation (9.7-9).
One thing's for sure: Mr. Hardly's perpetually nasty mood is definitely brought on by the harsh conditions of the Dust Bowl. In the same chapter as the change incident, Billie Jo informs us that he was dealt a major hardship when the store's attic filled up with dust and caved in. Most of his stock was ruined and it took days to repair it. So while he definitely seems to have a mean streak, it's not too hard to also feel a bit of sympathy for the guy.