Yakusugi is a mysterious priest figure rumored to be over 120 years old, possessing extreme physical and psychological capabilities that represent the pinnacle of human ability achieved through rigorous training. This status led to his selection as the test subject for the Dive Inspection experiment, which catastrophically failed. The failure resulted in his disappearance and the manifestation of Kankel, a destructive entity connected to all life on Earth. Despite this intrinsic connection, Yakusugi lacks the ability to control or contain Kankel.

Following the experiment's failure, Yakusugi periodically appears to aid the protagonists during critical moments, often providing unexpected assistance. He demonstrates significant power and is acknowledged as one of the most powerful humans. For instance, he intervenes during a confrontation with the creature Brahman but proves unable to defeat it alone, requiring intervention from Pakila, a powerful member of the Somnium race. His actions consistently position him as an ally against threats like Algernon and the Superhuman Federation.

Yakusugi's existence remains intrinsically linked to Kankel, dictating his ultimate fate. When Lamia successfully destroys Kankel, Yakusugi simultaneously perishes, a direct consequence of the failed Dive Inspection experiment that bound him to the entity. His physical appearance reflects his advanced age, characterized by gray hair, gray eyes, and shoulder-length hair, contributing to his senior and enigmatic presence.