Why does Harper Lee introduce the character of Lula in the story?
Harper Lee introduces the character of Lula in order to tell "the other side of the story," that there are tensions in the black community as well as in the white community. Lula represents those blacks who find the accusations against Tom Robinson very unfair--even offensive and malevolent, especially since they all know the Ewells who do not live far from the "black nest" as Bob Ewell calls it.
In Chapter 12 or To Kill a Mockingbird, despite some racial tensions because of the impending trial of Tom Robinson, Calpurnia brings Scout and Jem with her to her church on a Sunday that Atticus is gone. As they enter, the men remove their hats, and the women cross their arms before them in respectful postures.
But, suddenly out of nowhere, the children hear a voice, "What you up to, Miss Cal?" Her pose of a hand on one hip and palm of her other open in a point, indicates that she is not pleased. With Calpurnia's hand resting upon Scout's shoulder protectively, Calpurnia replies in "tones I had never heard her use," Scout remarks. In response to Lula's demand to know why Calpurnia has brought two white children to their church.
"They's my comp'ny," said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.
"Yeah, an' I reckon you's comp'ny at the Finch house durin' the week."
Lula hostilely moves forward, but Calpurnia stops her abruptly, speaking in the vernacular. Still, Lula is resentful, telling Calpurnia has no business in bringing white children to their church. But, Calpurnia replies wisely, "It's the same God, ain't it?"
In order to diffuse the situation, one of the male members of the church welcomes them.