Hamustra

Hamustra is the constellation who manages the Tower's floors from the 21st through the 30th, appearing as a small boy in oversized clothes who floats through the air. His name is a human approximation of an ancient being called "The Pond of Accumulating Memories," whose true name no human can pronounce.

He lived quietly among his stories until the Tower Master invaded his library and coerced him into serving as a floor-managing constellation. He curates "apocalypses"—nearly destroyed worlds and discontinued stories—and chooses warriors to save them. His secret dream was to appear inside one of these stories not as a manager but as a character. He also lived in fear of the Constellation Killer.

Everything changes when he meets Kim Gong-ja, who solves every trial set before him and treats the characters of his worlds with care. Gong-ja becomes Hamustra's favorite protagonist and helps unravel the Constellation Killer's grip on him. Eventually Hamustra breaks his contract with the Tower Master—losing his authority and becoming an ordinary person—to work at the Library Cafe Planetarium on the 30th floor.