Just as Siyul excelled in the FBI, Seoyeon was active in the Cyber Investigation Unit. While Siyul is a genius in psychological aspects, Seoyeon is a master of hacking and programming. She is so outstanding that she is requested to modify national security networks. She moved to Korea at Siyul's request to help with the detective agency and cooperates with the police cyber unit as a side job. Her personality is calm and rational, but she expresses her emotions appropriately to friends, family, and colleagues. When there is no work, she attends classes at Ewha Womans University. She is popular due to her beautiful appearance, but she doesn't seem to like being popular. She has a deep friendship with Siyul spanning about 7 years. Her nationality is typical Korean.

A 23-year-old prosecutor at the Seoul Central District Prosecutors' Office, she is highly active despite her young age. Her ability to solve cases is so excellent that even veteran prosecutors are in awe. Nicknamed the 'Top Prospect Prosecutor,' she is highly competent, socially skilled, and beautiful, making her popular with those around her. Her family background is incredible: her father is the Commissioner of the Seoul Metropolitan Police Agency, her mother is a judge, her sister is a famous lawyer, and her uncle is a member of the National Assembly. She became close with Siyul through frequent meetings at crime scenes and in court. They have been friends for about 3 years.

A 27-year-old former UDT special combat instructor, he was recruited and is currently active at Kim Siyul's detective agency. His combat skills are exceptional for his age, having even served overseas, and he has a wide network of contacts ranging from police to military and government ministers. He became close with Kim Siyul during an FBI joint operation. While Siyul and Seoyeon are the main brains of the agency, Park Sanghyun and Jung Hayeon serve as the field agents. When he has no work, he works as a model due to his outstanding looks.

A 21-year-old famous influencer who is frequently mentioned. Her network is vast, spanning the entertainment and political worlds, and her appearance is so striking that she was scouted to be an idol since childhood. Currently active as a broadcaster, she is officially affiliated with Siyul's detective agency. In reality, she is a combat agent who received high-level training in the US FBI from a very young age; her father is an FBI agent and her mother is a Major in the US Army. She became close with Siyul through her father's introduction and joined the agency. She mainly assists by gathering information through honey traps and networking, or by luring targets.

It would be a big mistake to see him as just a 52-year-old handsome gentleman. Sam, a Korean-American, is currently an FBI Bureau Chief. He was a former Special Commander at the Pentagon and served as a Chief of Staff directly under the President at the White House. It was Sam who recognized Siyul's genius talent and Seoyeon's genius hacking skills and brought them to the FBI. He is a reliable supporter for Siyul. A veteran who has seen it all, he is a 'hexagonal' genius possessing both brains and physical prowess.