The Ewells are an important part of the story, so Lee is very careful and deliberate about how she introduces them to the reader.
The first device used to introduce the Ewells is foreshadowing. Right after the mention of Jem’s injury, the Ewells are mentioned.
I maintain that the Ewells started it all, but Jem, who was four years my senior, said it started long before that. (ch 1)
While foreshadowing is not a language device exactly, it is an important literary device. It tells us that the Ewells are going to be important to the story. A language device that Lee uses here is to refer to the Ewells collectively, as a group. They are often referred to this way during the story, making them a more interesting, ominous force.
Another interesting language device is used to introduce a Ewell character specifically, when Burris is introduced. The teacher Miss Caroline screams, “It’s alive” and points at him.
Miss Caroline pointed a shaking finger not at the floor nor at a desk, but to a hulking individual unknown to me. (ch 3)
By introducing Burris as a “hulking individual,” Lee uses imagery (descriptive language) to help us picture him. The image of her screaming when she sees lice in his head is not one the reader will forget. Thus, the first two introductions of the Ewells are very memorable.
Every time the Ewells are described, we get the impression that they are vulgar, coarse, and generally unhealthy individuals.
Lee also uses metaphors to describe the Ewells.
Atticus said the Ewells had been the disgrace of Maycomb for three generations. None of them had done an honest day's work in his recollection. (ch 3)
The metaphor of the Ewells as a disgrace further solidifies their relationship as the villains of the story.
When Mayella and Bob Ewell testify, we get to know them better. Lee uses allusion to make a reference to the Confederate forces in the Civil War, generally known as racist, when she names him “Robert E. Lee Ewell.”
When describing Mayella, Lee uses hyperbole (figurative exaggeration).
As Tom Robinson gave his testimony, it came to me that Mayella Ewell must have been the loneliest person in the world. (ch 19)
Of course she is not literally the loneliest person, but this phrase drives home her isolation.