Evil season 2, episode 3 "F is for Fire" involves the case of alleged possession by an ifrit, an Islamic djinn, that appears to both a nine-year-old girl named Mathilde and investigator Kristen Bouchard. The end of the episode suggests that the possession is still unresolved and that the djinn has never left Mathilde. If this is the case, it suggests that the ifrit that Kristen sees is most likely a delusion caused by her own psychic distress.
"F is for Fire" focuses on the case of a mixed-faith family worried by their daughter's reports of seeing an ifrit and the fires that have been lit around their house. The episode expands the series' largely Christian mythology to includes elements of Islamic belief, furthering its dissection of contemporary religion that has earned Evil good reviews from critics. The episode culminates in simultaneous exorcisms from a priest and an imam, but ultimately neither seems to work. The episode ends with Mathilde sitting tranquilly on the lawn as a trash can lights on fire, suggesting that she is still being possessed. Later, in Evil season 2, episode 5 "Z is for Zombie," Kristen sees the ifrit while noticing ads from hook-up sites on her computer.
Evil is all about questions of faith and doubt, so there's no cut-and-dry answer for what Mathilde and her family are experiencing. There are multiple scenes of fires occurring near Mathilde without her seeming to light them or be near the site. She could theoretically have already lit a smaller fire in these locations, but the suggestion of demonic possession is hard to refute. Kristen only sees a fire-headed ifrit urging her to act recklessly, with no fires or other physical phenomena occurring around her. This means that Kristen's visions of the ifrit, like the demonic George, are more likely to be psychologically rooted.
It would be hard to pin down exactly how demons and similar spiritual entities work in the world of Evil, but so far there are no examples of one possessing multiple people at the same time. It was suggested that Kristen originally became demonically possessed after Caroline's exorcism, with the spirit going from Caroline to her, meaning that it couldn't possess two bodies at once. There are examples of phenomena that affect groups like the cursed song in "7 Swans a Singin," but no entities with personalities like demons or djinn. Even if the supernatural interpretation of events in Evil is correct, it seems unlikely that the ifrit actually possessed Kristen.
Kristen is also going through a number of psychological stresses that could cause her to hallucinate. Kristen murdered the serial killer LeRoux, or at least believes she did, and has been struggling to figure out how to deal with this. At the end of "F is for Fire," she sees the ifrit on her lawn before gulping down a fistful of pills. Its reappearance in "Z is for Zombie" shows that Kristen associates the ifrit with temptation and recklessness.
Like many other cases in Evil, Mathilde's possession by the ifrit isn't fully resolved at the end of the episode, with Kristen continuing to see the ifrit and the girl still lighting fires. The ifrit's brief appearance in a later episode suggests that this creature could be an ongoing plot thread. If so, Evil could shed further light on Kristen's visions of the ifrit and whether it is actually possessing her. But for now, it seems more likely that the flame-headed being is part of Kristen's ongoing psychological crisis.