In To Kill A Mockingbird, who are the Mrunas and who is J. Grimes Everett?

This is a good question. The Mrunas are an African tribe. J. Grimes is the missionary who is working among them. 

In chapter 24, Mrs. Merriweather is speaking to her missionary women's group about the work that J. Grimes is doing. On the one hand, this shows that Maycomb is a Christian town. It even shows compassion and charity to some extent. But the import of the chapter is really about the blindness of Mrs. Merriweather and the other women, who are representative of the people of the town.

Within this conversation, Mrs. Merriweather shows her true colors as a woman of little compassion and great blindness in her hypocrisy. I quote at length to show what kind of person she is.

Mrs. Merriweather faced Mrs. Farrow: “Gertrude, I tell you there’s nothing more distracting than a sulky darky. Their mouths go down to here. Just ruins your day to have one of ‘em in the kitchen. You know what I said to my Sophy, Gertrude? I said, ’Sophy,‘ I said, ’you simply are not being a Christian today. Jesus Christ never went around grumbling and complaining,‘ and you know, it did her good. She took her eyes off that floor and said, ’Nome, Miz Merriweather, Jesus never went around grumblin‘.’ I tell you, Gertrude, you never ought to let an opportunity go by to witness for the Lord.”

The juxtaposition shows that Mrs. Merriweather and the others in Maycomb are blind to their sins and issues. If they did a little more for their own community (instead of far off places), Maycomb would be a better place.