Kiryū Kusakabe is one of the nine central individuals in the narrative, distinguished by an air of profound mystery and a distinctly ominous presence. He is a man shrouded in enigmas, with nearly every aspect of his background, including his age, height, weight, and occupation, being unknown and his residence being transient. His appearance is that of a young man with dark brown hair and eyes, typically dressed in a black hoodie, grey pants, and black combat boots, a simple outfit that complements his shadowy nature.

Described by official sources as a "mysterious man" and by others as a "devil" who is "against everyone," Kusakabe's personality is defined by his unsettling and malevolent aura. He is characterized as someone who makes others feel uncomfortable, an ominous figure who seems to oppose everyone around him. His motivations are as opaque as the rest of his being; he appears to act on his own inscrutable whims, with one early description suggesting he might use others as tools until he grows bored of them.

In the unfolding events, Kusakabe serves as a crucial, if dangerous, agent. At one point, he has died and transformed into a malevolent ghost, yet he continues to interact with the living. In this spectral state, he becomes an assistant to Aria Kurenaino, a black magic proxy, aligning himself with her operations. His role seems to be one of execution, as he is implied to be responsible for bringing death upon clients requests, though he strictly maintains that he did not kill a specific target, Professor Isayuki Hashigami, despite being accused of breaking a "no killing" contract. This points to a complex code of conduct, even as he engages in death magic.

Kusakabe's most significant relationship is with Aria Kurenaino, whose assistant he becomes, though the nature of this alliance is ambiguous and potentially self-serving on his part. He is also notably linked to Ria Minase, meeting her as a ghost at the train tracks shortly after his own death, where she is distraught over her own brother's demise. Regarding his development, Kusakabe begins as a shadowy, living individual and is later explicitly shown to have died and become a ghost, a transformation that deepens his involvement in the story's paranormal core. Despite this shift in his state of being, his essential nature as an ominous and unpredictable force remains constant.

Perhaps his most distinct ability is that of astral projection, which he can perform to temporarily separate his spirit from his physical body, allowing him to perceive and communicate with other spirits. This power is intrinsically linked to his later status as a ghost himself, making him a uniquely versatile entity operating between the realms of the living and the dead.