According to the lore documents found on the Nightmaretaker fan wiki (which has over 3,000 pages of analysis), the demon inside Jonas Vellich operates on a unique rule set:

In the realm of horror and the supernatural, there exist tales that send shivers down the spine and make one question the very fabric of reality. One such legend is that of the Nightmaretaker, a man allegedly possessed by the devil himself. This dark entity is said to roam the earth, bringing terror and despair to those who cross his path.

Why is this horrifying concept suddenly considered attractive? We have seen possessed people before. They vomit pea soup, crawl on ceilings, and require priests. We do not find The Exorcist sexy.

The term "hot" in your query likely refers to the protagonist's design, which fits the "dark and mysterious" aesthetic popular in adult-oriented visual novels. Appearance:

He is the ultimate personification of the "I can fix him" trope: a man literally possessed by the devil, yet possessing a magnetic, brooding energy that has the entire internet in a chokehold.

The Nightmaretaker: The Man Possessed by the Devil. Contains erotic scenes with optical censoring The Visual Novel Database The Nightmaretaker: The Man Possessed by the Devil | vndb 22 Mar 2024 —

He retains his memories, his wit, and his tragic longing. But his shadow moves independently. His reflection whispers prophecies. And when he touches you, you feel two heartbeats: his, slow and sad, and the other, a frantic sledgehammer of pure malice.

And he burns for you.