Julia M. is the partner of Henry.
She meets him in a bar in Boulder, CO, in 1975 while Henry is drunk. He introduces himself and they hit it off. After getting married and a dog, Julia gets a job request at Yale University in Connecticut. However, she becomes increasingly unable to think straight, and it is revealed that she has early onset dementia, or Alzheimer's. Having been forced to leave her job, she enters full time care, which prompts her family to fly to the United States from Australia. Before long, she is taken back home to be cared for by her family as her condition worsens.
Julia is referenced by Henry many times within the dialogue of Firewatch, usually when Delilah asks him questions about her.
In a nighttime daze, Henry calls Delilah thinking it's Julia. Delilah picks up, only to hear Henry muttering in his sleep.