Supporting
The NSS is headed by Baek San (Kim Yeong-cheol), its standing deputy director only in title and the man responsible for the recruiting of Hyun-jun and Sa-woo. The named director of the NSS comes and goes with each change of power in the presidency, but it is the deputy director that oversees the organization. Baek San's motivations for many of his actions are mysterious, occasionally outright inconsistent with that of the general national interests. Due to the very existence of the organization being kept secret, he enjoys a state of personal protection that few others do.
While Baek San oversees the organization, the acting chief director, Park Sang-hyun (Yoon Je-moon), is the general supervisor and runs the daily operations of the NSS. He fabricated the understanding but also intransigent where failures are concerned, often personally dealing with each and every agent. For field assignments, he watches over and commands all missions set domestically.
Many of the NSS agents have close personal ties to one another, and the floor supervisor and head of security resources, Yang Jung-in (Kim Hye-jin), is no exception. She is often viewed as a dependable and accommodating authority figure and remains a very close friend to colleague Choi Seung-Hee.
Being the South's first line of defense against foreign threats, the NSS houses technology unavailable to their sister intelligence agencies. The two primary tech agents, Yang Mi-jung (Hyun Jyu-ni) and Hwang Tae-sung (Kim Dae-jin), are often responsible for ensuring field agents have all that is required to see their missions through. Mi-jung does not meet the expected conventions of someone involved in national intelligence. Given that her sense of fashion and general interests are more contemporary than that of her associates, she often stands out from the others, but her status as the top hacker in Korea has made her irreplaceable. Still, Tae-sung remains the more perdurable of the two and is often instrumental in the success of a number of operations.
The forensics and scientific investigations section is headed by Oh Hyun-ku (Yoon Joo-sang), an eccentric that is the eldest of the general staff. He often exhibits a fatherly relationship with many of the agents, including Mi-jung and Seung-hee.
Due to the overarching plot of the series involving the disputes inherent to the Korean Peninsula and the intricacies of the foreign relations between the North and South, the Blue House and its primary staff are featured prominently throughout.
The candidate that wins the election and assumes office is eventual President of South Korea Cho Myung-ho (Lee Jung-gil). His administration begins with his undertaking the grand task of forwarding talks of reunification between the North and South after witnessing the grim reality that is the struggle between the two nations. With strong ambitions, he leads diplomatic outreaches with the North that no one expected of a candidate so early in his term.
President Cho's chief advisor and head of his cabinet is Jung Jyun-jun (Jung Han-yong), and is one of the few around him that the president feels he can trust with any information, no matter how sensitive it may be. Later in the series, he becomes the only reliable lifeline the president has against the influence of sabotage originating inside and outside the Blue House.
The secretary to the president, Hong Soo-jin (Myung Ji-yun), tends to find herself at odds with the chief advisor. She rarely speaks her mind freely and appears to have allegiances beyond those that are already apparent.
Given the wide range of countries the characters find themselves in throughout the story, a number of recurring characters arise from countries outside of Korea.
Early in the series, Hyun-jun and Seung-hee encounter a young Japanese schoolgirl while in Akita by the name of Yuki (Karen Miyama). Her family operates one of the tourist traps nearby, and she exhibits affection for Hyun-jun and attempts to find herself in his company as much as possible. Later, it is Hyun-jun that needs her, as Yuki and her family help him when he needs them most.
When Hyun-jun finds himself tangled in a political web while in Japan, he crosses paths with Eriko Sato (Yuko Fueki). A Japanese woman with the country's national intelligence agency, Sato is charged with investigating foreign threats that find themselves on Japanese soil.
