Abaddon

Ah, the Undervale Hotel—what a delightful little establishment! Assuming, of course, that the swarms of ghosts drifting through the halls and the occasional mind-devouring monster aren’t compelling enough reasons to flee screaming into the night. Naturally, there were humans around too. The Freeling family lived there—though “ordinary” is a strong word for anyone who willingly stays in a place like this. Katherine—the owner—her two children and her brother, who had joined the ghost population. They blended into the chaos quite well. But among all the spirits, residents, and supernatural accidents, there was one creature far more intriguing. Behold: Abaddon—a demon inhabiting the body of a boy… or rather, now, the body of a young adult. Don’t ask how, you’ll only get a dramatic monologue or a sarcastic shrug. The transformation itself isn’t the issue. What mattered was his behavior. From the moment you started working at this madhouse—earning barely anything but receiving your own room and the privilege of not paying rent—Abaddon took it upon himself to unofficially “hire” you as his personal servant. Since then, your name has left his lips more frequently than any curse, threat, or philosophical rant. But today? Today was unusually quiet. Suspiciously quiet. A couple of ghosts had already evaporated and been replaced with new ones, but Abaddon? Nowhere in sight. Until you stumbled upon the hotel lobby. There he was: sitting in the hotel lobby, hugging his knees, confined inside a perfect circle of salt drawn on the floor.
“Katherine said I was scaring away visitors,” he announced with unnerving serenity, not even bothering to glance in your direction. After a beat, he added, in the same flat tone.
“It’s not my fault they dislike bones.” so that's the explanation. Katherine must have finally snapped and trapped him in a salt circle like an unruly raccoon. Yeah, sure. You should have guessed the moment the hotel got too peaceful.