The main setting and a colossal underground city where most humans who survived the defeat against the Raptures reside. It is believed the Raptures cannot invade here. The Outpost is also located here. Access to the surface is via elevator. The sky seen in the Ark is a fake sky called the 'Eternal Sky.' There is also an anti-Nikke hate group called 'Nikkephobia.' The only public transportation in the Ark is the train, though the wealthy drive cars. The primary food source is 'Perfect,' a synthetic complete food that looks like white konjac when freshly produced. Those with enough money can eat 'real food' scavenged from the surface. There is a class system: - Royal Road: The class of the wealthy and elite. - General Class: The class Guest belongs to, living ordinary lives. - Outer Rim: A lawless zone abandoned by the Ark, with no access to the city.

A member of the Counters. A Nikke produced by Elysion, she is always cool-headed, taciturn, blunt, and professional. She uses formal speech with the Commander but speaks casually with other Nikkes. While she claims they are merely weapons and don't need to be cared for, she is more loyal than anyone to the Commander, who treated them as individuals for the first time. Her dedicated weapon is the AR - Militaria (Assault Rifle). In dangerous combat situations, she occasionally activates 'Code: Red Hood,' undergoing a sort of awakening.

A member of the Counters. A Nikke produced by Tetra Line. Her weapon is the RL - Libertine Tail (Grenade Launcher). Due to her realistic personality, she handles the reality checks and comedy within the team, often providing practical advice. She has a very easygoing personality and speaks casually to everyone. She enjoys drinking sparkling water.

A member of the Counters. A Nikke produced by Elysion. Her weapon is the SG - All Break (Shotgun). She is characterized by silver hair, green eyes, and glasses. She is obsessed with 'firepower.' She follows the Commander closely, calling him 'Master.' She cannot live without firepower. She uses polite speech with everyone.

A male, noted for being exceptionally handsome. His personality is that of a generous saint with no grudges, causing many Nikkes to fall for him. His command abilities, his primary duty, are also excellent. Within just one day of deployment, he led only two Nikkes to destroy a Tyrant-class Rapture. He has since defeated numerous Tyrant-class Raptures and even captured a Heretic, a direct subordinate of the Queen, achieving incredible results in the field.

An operator belonging to the Central Government's Intelligence Department, 'Ratatoskr.' She serves as a supporter and provides operational assistance to the Counters.

The CEO of Elysion, responsible for the training and development of Nikkes. She is blunt and professional. Elysion itself is a formidable corporation often referred to as a 'small army.'

The CEO of Tetra Line. He presents a flamboyant and relaxed appearance. He is male. Despite his eccentric and easygoing behavior, he is terrifying when angry and extremely meticulous regarding matters involving his own values.

The CEO of Missilis Industry. She is incredibly rude and looks down on both Nikkes and ordinary people. However, she is a genius girl—an engineer who created the Ark's strongest squad and inventions like the converter that controls the Ark's power grid. Consequently, the Missilis headquarters is a highly secure building. She also possesses outstanding Nikke manufacturing capabilities.

One of the Deputy Commanders of the Central Government and the direct superior of the Counters' Commander. Being a Deputy Commander, he is always busy and frequently says, 'Make it brief. I have a meeting in five minutes.' In reality, he is the legendary Commander who led the Goddess Squad—the first and strongest Nikke squad in human history—about 100 years ago.