Some Christians hold that the universe or creation are part of God (panentheism). More often than not, God is said to be both immanent and transcendental in relation to the universe. How can those be accepted if Satan is to be treated as a real entity? While immanence may allow you to say that God pervades being but is external to it, panentheism allows no such move. It would imply there is no "other" in relation to God, only but God and parts of God. And as soon as you introduce parts, you immediately wreck the supposed simplicity of God.
The answer can be seen in YHWH's omnipresence. Understanding a being truly being in ALL places as once, as omnipresence demands, leads to a line of thought that is not at all intuitive. Omnipresence requires a being to be present in both a macrocosmic sense as well as a microcosmic sense. Or to put it another way, YHWH is simultaneously and equally present in a single carbon molecule, a rock, a person, a planet, and a galaxy. Omniscience explains how YHWH is capable of perceiving on so many levels at once.
To be clear, omniscience treats that carbon molecule or person as a place, not an object or living being. That allows for a being to be present in a place that has its own distinct identity. Illustrating the point, if you have a flea on top of a dog, the flea is treating the dog as a place, despite the canine's nature as a distinct organism. (I am not attempting to compare YHWH to a flea here, except in an analogical sense.)
Here's where it gets rather tricky and debatable. If YHWH is present in all places at once, he is both in Everything and Nothingness. (Debatable because of a popular theory that suggests Nothingness is merely the area He chooses not to dwell).
Now to include Satan in the mix. Yes, omnipresence demands that YHWH is present in Satan as well. The free will argument explains how a being of absolute goodness can be present in a being of absolute evil. Barring that, however, is the theory that stems from Satan seeking YHWH's permission to tempt Job; that even what Satan does is done through divine will. I won't attempt to argue one of those alternatives over the other.
Those who would hold to some sort of panentheism would be oldschool theological liberals. They are a rapidly disappearing breed and most conservatives would not grant them Christian recognition anyway.
Elements of pantheism creep into Christian doctrinal statements mainly when theologians want to eliminate supernatural elements from their beliefs. Thus by the time they get to thinking about Satan, they have already said that Jesus isn't divine in any sense we are not divine. I don't think it give them any heartburn to say that Satan is a figurative/mythical character.
Short answer: If you have allowed elements of panentheism in your theology, you've got much bigger problems than how to explain Satan.