Technically, four characters are named in the story: Mrs. Turpin, Claude, Mary Grace, and Miss Finley. However, Miss Finley is a character with a momentary appearance of no consequence whatsoever, so this answer will focus on the first three. The titular revelation that occurs in the story is that the preconceived notions about class and status that Mrs. Turpin perceives as virtues are at best misguided and at worst deeply prejudiced and vain.
The story is written in free-indirect style from Mrs. Turpin's point of view. Characters other than her beloved Claude are referred to by their most superficial characteristics. Mrs. Turpin's view of the world revolves around her perceived hierarchy of human value. She originally places herself somewhere in the middle of her imagined pyramid, though she is able to make exceptions about people who own more than her, arriving at a viewpoint that allows her to look down on anyone.
In fact, before her mother reveals the name of Mary Grace, Mrs. Turpin refers to the character only as the "ugly girl." It is interesting that that her name is so connected with Catholicism, being that Flannery O'Connor was a devout Roman Catholic herself. Mrs. Turpin is horrified that the message Mary Grace gives her is a message from God himself. Her vision at the end of the story reveals all the people she has looked down upon getting into heaven before her.