Revisit the earlier character's backstory and intersperse it with dialogue.

"Who are you and what are you doing here?" she asked, after a time. Grandmother was not a hard person, though the war had made her resilient.
"Aggie..." was the only response the stranger could muster. But to her, there could be no word more telling. She was not accustomed to being address so, had not been for forty years. Her husband had called her Agnes, she could remember him calling to her on the day he died. Another life, it seemed. The life of Aggie, yet another. But that was what he had always called her, something she resented him for at first.
They stood in silence, Grandmother not daring yet to acknowledge him. Finally, he said again, "Aggie. It's me. Did you not get my letter?" There was something there, she thought. Something in the voice. Which was strange, because the accent was German now, and carried the huskiness of old age. But a tonal quality, something unmistakable, was familiar. The last time she had heard it was so long ago. A lifetime ago - hers and her husband's. She had said goodbye to it and had never thought she would hear it again. Now it was in her front room.
And before she knew it, there were tears in her eyes. She was not immediately aware of a sensation of sadness. But the body remembered what the mind could not grasp, and it wept. It wept for the report of his death in France, it wept for her loneliness, it wept for her husband, who had lifted her from that misery. And now it wept because the boy who died forty years ago was now a man in her hallway, with the same light in his eyes.