Chris Keller and Ann Deever grew up as next door neighbors, their houses were side by side on the same block. When they were younger, Ann was Chris's brother Larry's girl, in fact Ann was engaged to marry Larry before he went off to war. Now that Larry has not come back from the war, he is missing in action from the beginning of the play, Chris and Ann have been writing letters to each other for three years.
Ann moved to New York City with her mother when her father, Steve was sent to prison for the crime he committed with Joe Keller, shipping faulty airplane parts to the military. Now, Chris has written to Ann and asked her to come to his home, which I believe is in Ohio, because there is a reference that they are about 700 miles apart, because he intends to ask her to marry him.
Chris and Ann are in love with each other and Ann desperately wants to marry him, she wants to move forward with her life and for most of the play, so does Chris. Kate Keller is strongly opposed to Ann being in the house, and is totally against Chris marrying Ann, who for her will always be Larry's girl.
Kate's opposition towards Ann stems from her belief that as long as Ann continues to wait for Larry, then she strongly believes that he is alive, as Kate wants to believe.
Chris and Ann don't end up getting married in the play, Chris, once he finds out that his father is guilty and a liar, he can't marry Ann, because of what his father did to her father.