The Wall of Severance is a barrier that separates humans and bears in the atmosphere. It is created to prevent bears from attacking the human world - Earth.
Long ago, a far off planet known as Kumaria exploded, turning into a meteor shower that fell upon earth. This lead to bears becoming violent and attacking humans, who created the Wall of Severance to separate humans and bears. However some bears have disguise themselves as humans and successfully entered human earth. Example being Lulu Yurigasaki and Ginko Yurishiro.