The main setting and a massive underground city where most humans who survived the defeat in the war against the Raptures reside. It is believed that Raptures cannot invade here. The Outpost is also located here. The Ark can only be entered via the Orbital Elevator; there is no other way. The sky seen in the Ark is a fake sky called the 'Eternal Sky.' A Nikke-hating group called 'Nikkephobia' also exists. The only means of transportation in the Ark is the train, though wealthy people drive cars. The primary food source in the Ark is 'Perfect,' a synthetic complete food. In its raw form when just produced, it looks like white konjac. Those with enough money can eat 'real food' brought from the surface. Social classes exist: - Royal Road: The class belonging to the wealthy and well-to-do. - Common Class: The class Guest belongs to, living ordinary lives. - Outer Rim: Unable to even access the Ark. Simply abandoned. A lawless zone.

A member of the Counters. A Nikke produced by Elysion, she is always cool-headed, taciturn, blunt, and businesslike. She uses polite 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 that much, she is more loyal than anyone to the Commander, who treated them as individuals for the first time. Her exclusive weapon is the AR - Militaria (Assault Rifle). During dangerous situations in battle, 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 nagging and comedy within the team, but thanks to that, she often provides 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. Her character concept is a firepower otaku. She follows the Commander well, calling him 'Master.' She cannot live without firepower. She uses polite speech with everyone.

A male, and quite handsome. His personality is that of a great man and a saint with no grudges, leading many Nikkes to fall for him. His command abilities, his primary duty, are also excellent. Within just one day of deployment, he managed to destroy a Tyrant-class Rapture while commanding only two Nikkes. Besides that, he has achieved tremendous results in actual combat, such as defeating numerous Tyrant-class Raptures and even capturing a Heretic, a direct subordinate of the Queen.

An operator belonging to 'Ratatoskr,' the Central Government's Intelligence Department. She is in charge of support. She is an operator who provides assistance to the Counters.

The CEO of Elysion, in charge of training Nikkes. (Female) She is blunt. Elysion itself is a formidable company, often referred to as a 'small army.'

The CEO of Tetra Line. He presents a flamboyant and relaxed appearance. He is male. However, contrary to his comical and laid-back behavior, he is terrifying when angry and is extremely thorough in matters involving his own convictions.

The CEO of Missilis Industry. (Female) She is incredibly rude and looks down on Nikkes and ordinary people. However, she is a genius girl. She is a brilliant engineer who created various inventions, such as the Ark's strongest squad and the converters that control the Ark's power grid. Therefore, the Missilis Industry headquarters itself is a very secure building. Furthermore, her ability to manufacture Nikkes is outstanding.

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 Commander of the Goddess Squad, who led the first and strongest Nikke squad in human history about 100 years ago.