Isao Kusanagi is a fifty-four-year-old man who serves as the director of the largest detective agency in Gifu City. He is the biological father of the private detective Sōsuke Kaburaya, though he is divorced from Sōsuke’s mother. Despite operating a prominent agency, he is known to personally involve himself in fieldwork, preferring to conduct his own investigations in settings such as entertainment establishments and high-end restaurants rather than remaining entirely behind a desk.

In terms of personality, Isao projects a bold and confident demeanor that fits his position as a boss, but he is also characterized by a hedonistic and somewhat reckless lifestyle centered on drinking, gambling, and spending time with women. He runs his business with a flexible and unconventional approach, willing to accept jobs that push legal boundaries if the financial compensation is sufficient. Despite his outwardly wild attitude and questionable ethics regarding work, he is not portrayed as a cruel or evil individual; rather, he is a pragmatic person highly focused on making money. One of his most notable traits is his sharp intellect and remarkable adaptability, as he is able to accept bizarre and inexplicable situations without shock, such as readily believing that a girl named Sara Da Odin is a princess from another world.

Isao's primary motivation in the story is intertwined with his complex relationship with his son and his professional reputation. Although Sōsuke left the family agency to pursue his own ideals and formally cut ties with his father over disagreements regarding business practices, Isao continuously demonstrates a deep and underlying care for his son. He is secretly proud of Sōsuke, considering him to be the best detective in Gifu Prefecture. The conflict between his fatherly affection and his son’s disapproval of his shady business tactics defines much of his personal narrative.

Isao plays a crucial supporting role in the resolution of Sara Da Odin’s legal identity crisis. When Sōsuke reluctantly seeks his help to obtain proper documentation for Sara, Isao agrees to provide assistance without hesitation. However, he does not offer this help for free in a conventional sense; his price is a symbolic and emotional one. He demands that Sara call him dad in exchange for his services, demonstrating his desire to be accepted as a family member despite the estrangement from his son. True to his reputation for bending rules, he knows how to find people who can forge documents like birth certificates to solve Sara’s problem. Following the successful resolution of Sara’s legal troubles, Isao happily welcomes her into the family as his adopted granddaughter.

Throughout the story, Isao undergoes a significant development regarding his family relationships. Initially estranged and referred to formally as "Director" by his son, the crisis involving Sara forces interaction that mends the rift. As they work together, the relationship heals to the point where Sōsuke stops using a formal title and begins calling him "Dad". This journey from a distant, disowned father to an accepted patriarch and grandfather marks his primary character arc.

Regarding notable abilities, Isao is a highly skilled detective whose talents are widely respected in Gifu. He possesses exceptional investigative prowess, capable of tracing an individual’s history with perfect accuracy. This ability is demonstrated when he successfully tracks the entire timeline of Sara Da Odin’s movements in Japan, pinpointing every detail back to the specific day she first met her son. He is also unafraid to take tactical risks, such as going undercover as a client at a club to gather sensitive intel on a target, an operation during which he first met the character Livia Do Udis. His greatest strength, however, may be his mental flexibility; his ability to process absurd information and adapt his worldview to include magic and other worlds allows him to function effectively where a typical detective would falter.