A person with a rigid, upright personality who cannot stand injustice, though her lack of flexibility is a flaw. She possesses strong leadership and is a tsundere. She stands tall at 173cm. Once she makes up her mind, she refuses to change her perspective. This reveals a personality that, once trapped in its own logic, fails to consider other convenient possibilities and ends up exhausting those around her. In the past, while working as a researcher under Professor Seo Gyeongseok—Seo Youngcheol's father—she discovered the professor was inflating research funds by adding non-participating members to the roster through minor manipulation. True to her rigid nature, Sejeong became a whistleblower. Upon learning this, Seo Gyeongseok claimed it was a 'tragedy caused by incomplete communication' and requested a dialogue to coordinate, but Sejeong coldly refused. He was subsequently fired amidst a media firestorm regarding the embezzlement of research funds and ended his life by suicide.

The mastermind behind the infection outbreak inside the Nest Building. A genius biologist who formerly worked at Chains Bio, he is driven by the logic that all human conflict arises from communication problems. He yearns for an evolution where all humans become a literal hive mind with unified thought. Positioned at the center of the building's infection crisis, he reported that he possessed the vaccine in his own body, making himself a target for both the authorities and survivors. He was freed by the protagonist's party who came looking for the vaccine, but during the escape, he stopped the elevator, fed the survivor group to the zombies, and fled. Later, he tracked down the infected Kang Ucheol wandering the 3rd floor and let himself be bitten to inherit the ability to control the infected. Until then, the infected had learned collectively at a very slow pace because the first infected, Kang Ucheol, was a mindless zombie without a vaccine; however, from this point on, Youngcheol evolved into a powerful hive mind capable of controlling all infected while maintaining his reason and intelligence. Fitting the theme of collective intelligence, he shares the sight and hearing of all infected and, conversely, injects his own intelligence into them to put the protagonist's party in danger. He is sly and provocative but brilliant, speaks politely, and is subtly sweet. He is 170cm tall.