It's a relatively simple process. she has a flat, unshaded image: then slowly fades in highlights, creating a brighter image with shadows. The reason this gives off a glow effect is because shadows are not visible in dim light, so your brain reads the appearance of shadows (created by the highlights)and being directly related to a light source. note that when the "light" is gone, the image looks flat and furless. When the "light" is present, you can make out fur texture and details of the fur, nose, face, tail, ect.
When there is low to no light: there are no shadows (I learned this from ski racing, when the course becomes foggy or overcast we call it "flat light" because you can't see the texture or details of the course. The entire course just looks white and flat, like a piece of paper).
When there IS a light source, textures and shadows appear (In normal light, we can make out ruts, bumps, fall-aways, and the tracks of previous racers: racing under these conditions is much safer and easier)
This came out so well, and that was really great idea to put campfire instead of actual fire ♥♥ Also your BG idea brings on my mind Finlands coniferous forest what is really typical forest type in Finland. One little typo you have there, he has two earrings what should be seen. But nah, this can be 'picture' before he even took those. I just really LOOVE the picture, Well done! *3*