only exists in Harry's brain

Lasciel, aka the Temptress, aka the Webweaver, apparently some kind of photocopy of the personality of a fallen angel, sat in the passenger seat. She could look like anything she chose, but her most common form was that of a tall, athletic blonde wearing a white Greek-style tunic that fell almost to her knee. She sat with her hands in her lap, staring out the front of the car, smiling very slightly.
"What the hell do you think you're doing?" I snarled at her. "Are you trying to get me killed?"
"Don't be such a baby," she replied, her tone amused. "No one was harmed."
"No thanks to you," I growled. "Put the seat belt on."
She gave me a level look. "Mortal, I have no physical form. I exist nowhere except within your mind. I am a mental image. An illusion. A hologram only you can see. There is no reason for me to wear my seat belt."
"It's the principle of the thing," I said. "My car, my brain, my rules. Put on the damned seat belt or get lost."
She heaved a sigh. "Very well." She twisted around like anyone would, drawing the seat belt forward around her waist and clicking it. I knew she couldn't have picked up the physical seat belt and done that, so what I was seeing was only an illusion-but it was a convincing one. I would have had to make a serious effort to see that the actual seat belt hadn't moved.
Lasciel looked at me. "Acceptable?"
"Barely," I said, thinking furiously. Lasciel, as she appeared to me now, was a portion of a genuine fallen angel. The real deal was trapped inside an ancient silver denarius, a Roman coin, which was buried under a couple of feet of concrete in my basement. But in touching the coin, I'd created a kind of outlet for the demon's personality-embodied as an entirely discrete mental entity living right in my own head, presumably in the ninety percent of the brain that humans never use. Or in my case, maybe ninety-five. Lasciel could appear to me, could see what I saw and sense what I sensed, could look through my memories to some degree and, most disturbing, could create illusions that I had to work hard to see through-just as she was now creating the illusion of her physical presence in my car. Her extremely attractive and wholesome-looking and entirely desirable presence. The bitch.


Proven Guilty Chapter 3, Page 16