The Ewells are one of the poorer families in Maycomb. Mr. Ewell is often referred to as the town drunk. He cannot keep a job and does not support his family. His eldest daughter, Mayella, is often left with the task of caring for the family. The Ewell's are often looked down on or pitied because they are uneducated and often unclean. If Mr. Ewell tried to support his family instead of drinking, the town's opinion of him and his family might be different.
This family plays a key role in the story because it is Mayella that accuses Tom Robinson. It is this accusation that sparks a trial and many other difficulties for the small town. Atticus and his family are treated differently because he defends Tom in court. We see that even though the town's people do not like the Ewell's, they will certainly take their side over that of a colored man. This shows just how deep the prejudice and discrimination runs.
Bob Ewell also plays a key role at the end of the story. It is his attack on Scout and Jem that reveals the true nature of Boo Radley. We have seen hints of it throughout the story, but this final act of rescue seals the reader's opinion.
While the Ewell's may seem like secondary characters. They play an important role in many aspects of To Kill a Mockingbird.