They were the dominant race among the outcast and they mistreated several of the other races. They especially mistreated the Dooni, which they actually massacred all of them except one, Devy, was treated like a slave. After Devy got the hammer from Yeon Woon most of the Khukum were killed by Devy, leaving only a few on the 49th floor.
Appearance and Abilities
The Khukum apparently look very similar to humans, but due to the hazardous environment of the 49th floor they wear protective gas mask that cover their whole face.
They have similar levels of intelligence to humans and similar level of abilities, which would not be very impressive if they climbed the tower, but it made them the most powerful among the outcasts.