Xaanik

• Any pronouns
• Looks however you'd expect her to look. If you asked 10 artists to draw her, you'd get 10 completely different drawings of 10 completely different people, no resemblance whatsoever. And yet anyone who looked at the pictures would agree they depicted the same person. Xaanik completely destroys all cognitive inconsistency by the nature of her existence.
• At least as old as thought. Unicellular, that is.
• A forgotten fragment of divinity
• A figment of the collective imagination
• Functions according to the logic of the beholder's own mind
• Can induce any mental state; suppress or induce any thought. Cognitively omniscient and omnipotent. Knows you better than you will ever know yourself, and sees through your deepest self delusions. Anything which is even slightly alive is fair game for her. She can make any living thing do anything, including magic things.
• Case in point: Sells/hands out enchanted seeds which have magic effects. The way she does this is by hypnotising the seeds into thinking they have powers, and she's so convincing that they and the rest of the universe believe it
• Does not exist to beings from beyond our own universe. Extradimensional beings will instead see the "ground truth"—a world with no Xaanik. If xaanik handed you an apple... What the Extradimensional Being will see is: no she didn't, you picked it up yourself. They can still hear about mentions of her, but if someone told them "Xaanik is sitting next to you right now" they'd just have to take it as metaphorically as if you'd suggested jesus was doing the same. Her powers will have no effect on extradimensional beings, but nor can they effect her.
• Xaanik the thing which eats all your lost socks and forks and tupperware lids.
• Does actually exist, even though no she doesn't. Despite seemingly existing to do only the will of those she encounters, she does have a will of her own. It's just that it is completely ineffable and cosmically horrific. You cannot know what she is or why she does anything :) but judging by her behaviour, she is generally, though not entirely, benevolent.
• Completely unkillable, so long as life exists in the universe. Even that's not a guaranteed end. She may or may not exist more like a fundamental particle than a symptom of life.

She'll tend to vindicate you on whatever it is you truly desire. Case study, so you understand how horrifying her influence really is: 

Neither of these recounts are any more or less true than the other, because that's just how Xaanik works. For what it's worth, as far as anyone in-universe is concerned, the true reality was the one on the left. If it hurts your brain, it's supposed to.