The Conjuring: The Devil Made Me Do It is based on the real legal case of Arne Johnson Cheyenne Johnson, but the film also explores the more personal relationship between him and his girlfriend (and, later, wife) Debbie Glatzel. Although like the rest of The Conjuring movies, the actual plot strays away from the real-life story in favor of theatrical scares, Debbie's character arc is, in fact, a fairly accurate reflection of her actual involvement in the events.
The Conjuring 3 mostly focuses on the supernatural horrors that occur after the possessions of Debbie's younger brother, David Glatzel, and, subsequently, Arne Johnson. Debbie herself is present at the time of the murder and defends Arne throughout the trial, siding with the Warrens and claiming that her boyfriend killed his landlord Bruno Sauls, whose real-life counterpart was named Alan Bono. She remains by Arne's side as much as she can while he's in custody and eventually marries him during his eventual prison sentence.
In the true story of The Conjuring 3's Arne Johnson case, Debbie factors into the murder in a bit more complicated manner. For starters, she wasn't the only one on the scene. Arne's sister Wanda and, more significantly, Debbie's nine-year-old cousin, Mary, also accompanied the couple and the victim that day. The situation escalated when Bono, who had been drinking during a lunch he had with the group, grabbed hold of Mary's arm and refused to let go. Debbie reportedly tried to intervene between the two men, who grew aggressive towards each other, before Arne stabbed Bono with his knife. Debbie, now a witness to the murder, claimed that Arne was making growling noises and was caught in a trance-like state when committing the violent act. Her report on Arne's strange, animalistic behavior led Debbie to insist that her boyfriend was, in fact, possessed by demons.
In the movie, the relationship between Debbie and Arne is painted as a reflection of the Warrens' love for each other. In both cases, that bond is what centers these otherwise pure-hearted individuals amongst all the malevolent curses and murderous devils. That romantic portrayal may seem exaggerated, but in many ways, it's one of the most accurate aspects of The Conjuring 3. Debbie really did marry Arne in January 1984 while he was still behind bars, and she continued to proclaim his innocence right up until her recent death (exact date unknown) from cancer (exact type unknown).
Not much else is known about the life of Debbie Glatzel, especially after her marriage. What is known is that the bond between Arne and Debbie started early in their lives, when Debbie met a 12-year-old Arne at a supermarket, and that their relationship hardened as the couple helped take care of Arne's siblings, as well as his sick and ailing mother. Unlike her brothers David and Carl Glatzel, the latter of whom wasn't even included in the movie, Debbie has consistently claimed that the demonic possessions of both David and Arne were true, and that Arne exhibited strange behavior that included growling and saying that he saw "The Beast" in the weeks leading up to the murder. Debbie was also apparently involved in the making of The Conjuring: The Devil Made Me Do It before she died, explaining her movie character's accuracies.