Who is Mr. Simms, and what role did dhe play in Chapter Five of Roll of Thunder, Hear My Cry?

You didn't specify which chapter in your question so I
have gone for Chapter Five of this great novel, which shows Cassie's confrontation with
the Simms family after she accidentally bumps into Lillian Jean Simms. Mr. Simms is a
white man who has land close to the Logans. Jeremy, his son, likes to walk to school
with the Logans even though his father would not allow it if he knew about it. It is
clear however from his actions in this chapter that he is very racist and believes that
whites are superior to blacks in ever way. He demands that Cassie apologises to his
daughter, callling her "Miss Lillian Jean," ad supports his daughter when she tells
Cassie to get off the sidewalk and walk on the road itself. Note how Cassie describes
him:

Then Mr.
Simms jumped into the street. I moved away from him, trying to get up. He was a
mean-looking man, red in the face and bearded. I was afraid he was going to hit me
before I could get to my feet, but he
didn't.

Mr. Simms is
therefore a character that forces Cassie to realise the discrimination that exists in
the world and how unfair life is. He manages to turn what was an event Cassie was
looking forward to, travelling to market with her grandmother, into an immensely sad and
tragic event as Cassie is forced to confront the rampant inequality that exists in the
world.