What is upper moon 4?

Upper Moon 4, also known as Rui, is a character in the anime series Demon Slayer. Rui was a demon who was previously a human child, but he was turned into a demon by Muzan Kibutsuji. As an upper moon, Rui had tremendous strength and agility, and he was also blessed with the ability to create webs to trap his opponents.

Rui was known for being a cruel and sadistic demon who enjoyed torturing his victims. He created his own "family" of demons, which consisted of other children that he had turned into demons. Rui was particularly obsessed with finding a "sister" to add to his family, and he was drawn to Nezuko Kamado, the sister of the protagonist, Tanjiro.

In the series, Tanjiro and his allies fought against Rui and his family of demons. Rui was eventually defeated by Tanjiro and his sister, who had managed to resist the demon's control and become a demon slayer. Despite his cruel nature, Rui's defeat was portrayed as tragic, as he had been a lonely victim of Muzan's cruel machinations.