Asuha Chigusa is the younger sister of Kasumi Chigusa and serves as the student representative, or head, of the Chiba coastal defense city. She is a girl with long, messy hip-length red hair that she typically ties back with a black hairband, and she has red eyes. Her usual attire includes her uniform with a pink jacket wrapped around her waist. Asuha is known for her cool, laid-back, and tomboyish personality, often appearing listless or lethargic in her daily life. Despite this calm exterior, she becomes entirely different the moment she picks up a weapon, transforming into a trigger-happy fighter who is enthusiastic about engaging in combat.

Her primary weapons are dual-wielded pistols. Asuha possesses a supernatural ability known as a World, which grants her the power to control the movement of material objects. Her power is considered exceptionally strong, allowing her to engage enemies directly without relying heavily on complex strategies. This strength enabled her to rise quickly through the military ranks of Chiba, becoming the city's third-in-command even before entering high school.

Asuha's relationship with her older brother, Kasumi, is a central part of her character. She often acts coldly toward him and has a habit of pushing him around, but this behavior masks a deep and genuine care for his well-being. This protective instinct is demonstrated in how they came to lead their city. Originally, their tactical skills led to Kasumi being nominated as the city head, but he was exhausted from overusing his abilities. To protect him from the dangerous front-line duties expected of a leader, Asuha publicly defeated him and declared herself the strongest, thereby taking the position of city head for herself. This left Kasumi to act as her second-in-command and de facto strategist, handling the administrative work while she took on the more direct combat responsibilities. Their sibling dynamic is a notable part of the story, balancing her bombastic and controlling nature with his lazy and cynical disposition.

Her motivations are deeply tied to her brother and her role as a protector. While she might express a desire to avoid troublesome situations, her actions consistently show a commitment to fighting for her city and for Kasumi. Over the course of the narrative, Asuha, like the other main characters, faces a major revelation that challenges her understanding of the war against the alien invaders known as the Unknown. After discovering that the enemies they have been fighting are not what they seem, she ultimately reunites with her mother and joins the effort to rally the remaining children for a final confrontation against the true threat.