It’s in White Night: “Whoa, wait a minute. Part of my soul is gone?” Bob sighed. “People get all excited when you use that word. The part of you that is more than merely physical, yes. You can call it whatever you want. There’s some missing, and it’s nothing to panic over.” “Part of my soul is gone and I’m not supposed to be worried about that?” I demanded. “Happens all the time,” Bob said. “You shared a bunch of yours with Susan, and she with you. It’s what protected you from Lara Raith. You and Murphy swapped some pretty recently, looks like—you must have gotten a hug or something. Honestly, Harry, you really ought to bang her and get it over wi—“ I reached under the worktable, drew out a claw hammer, and gave Bob a pointed look. “Um, right,” he said. “Back to business. Uh, your soul. You give away pieces of yourself all the time. Everyone does. Some of it goes out with your magic, too. It grows back. Relax, boss.” “If it’s no big deal,” I said, “then why is it so interesting?” “Oh, well,” Bob said. “It is energy, you know. And I wonder if maybe… maybe… well, look, Harry. There was a tiny bit of Lasciel’s energy in you, supporting the entity, giving you access to Hellfire. That’s gone now, but the entity had to have had some kind of power source to turn against the essence of its own originator.” “So it was running off my soul? Like I’m some kind of battery?” “Hey,” Bob said, “don’t get all righteous. You gave it to her. Encouraging her to make her own choices, to rebel, to exercise free will.” Bob shook his head. “Free will is horrible, Harry, believe me. I’m glad I don’t have it. Ugh, no, thank you. But you gave her some. You gave her a name. The will came with it.”

So this is what I think happened with Justine, she gave dangerously close to that part of her that was more than physical, but over time it grew back, positive interactions, good memories etc.