King Paimon is an actual demon. He's named specifically in the Ars Goetia section of the Lesser Key of Solomon and the Dictionaire Infernal, and most of the other official sources of recognized demons and spirits.
That said, Stephanie doesn't know shit about him beyond the movie, and couldn't call a demon if she tried. Contrary to unpopular beliefs entirely made up by Stephanie, demons don't work for free, for chocolate, for plastic beads, for table scraps, or for dead foot skin, spit, waste blood, or piss.
She's a complete imbecile and if she ever did manage to actually contact a demon, she'd find herself possessed, tormented, and dead in the space of 2-3 weeks.