These two white landowners never get any screen time, but other characters refer to them several times.
Mr. Montier owns a lot of former plantation land in the area, and is quite ruthless (along with Mr. Granger) with the families who rent his land. After he finds out about the boycott of the Wallace store, he makes his sharecroppers pay him a larger percentage of their cotton crops than before.
Not Mr. Harrison. He deals fairly with the African-American families. Mr. Avery describes him as a "decent man" (9.72).
Mr. Grimes is the school bus driver for Jefferson Davis County School. It's fitting that he has a grimy name, since he's a slimy, grimy man; he finds it hilarious to splash the black children with mud and dust every day with his school bus by barreling down the dirt road while carrying the white children to school. He gets his in the end, though, when the Logan children sabotage the bus and he makes the first-hand acquaintance of the mother of all potholes, putting his precious bus is out of commission for at least two weeks.
The sheriff doesn't even have a name in the novel, so he's here pretty much as a figure for corrupt justice. Throughout the story, he lets the white mobs get away, literally, with murder. The book strongly implies that he's in Mr. Granger's pocket and does what he says. He's willing to let the mob carry T.J. off for lynching—just as long as they don't do it on his land (check out the "Justice and Injustice" section in "Themes" and you'll see what we mean).