J. Grimes Everett is mentioned several times in Chapter 24, but he never makes an actual appearance. Everett is a missionary who has traveled to Africa to live with Mruna tribe, and Mrs. Merriweather met him at a church camp. The purpose of the Missionary Circle tea is to discuss the matter of the Mrunas (and probably drum up some financial support for Mr. Everett). According to Mrs. Merriweather, Everett claims that the Mrunas (who are non-Christian) live in "sin and squalor." So she has pledged to deliver his message to the Missionary Circle. Everett is more than likely a white Methodist minister.
J. Grimes Everett is mentioned in chapter 24. He is a missionary in Africa. According to Mrs. Merriweather, he is working among the Mrunas, who are living in "sin and squalor."
He does not play a role in the book in a direct way. The purpose of introducing him is to show the blindness and racism of Mrs. Merriweather and other inhabitants of Maycomb.
From the surface, all seems well in Maycomb. It is a sleepy Southern town, and the people are Christian and they even care about missionary work in Africa. However, as we look deeper, there is racism, hatred, and feelings of arrogance. If Mrs. Merriweather really wanted to help blacks, then should would do something about the situation in Maycomb. She would stand up for innocent men, such as Tom Robinson. The fact that she did none of these things says that she is hypocrite.
So, J. Grimes Everett is in the novel to expose the real heart of Mrs. Merriweather and many others in Maycomb.