Kaoru Asao is a high school student and a supporting character who serves as a classmate and close friend to the protagonist, Shio Ogura. She is a girl of average build with fair skin and long, dark green hair that reaches her waist, paired with yellow eyes. As a student, she is typically seen in her school uniform.

In terms of social standing, Kaoru is depicted as a popular and friendly individual. She is outgoing and does not appear concerned with conforming to social hierarchies, as she is the one who approaches the more isolated Shio during a school trip to Taiwan. Her initial action is to invite Shio to join her group for sightseeing, an offer born from genuine kindness rather than social calculation. When Shio hesitates because she believes a popular girl should not be seen with an outcast, Kaoru disregards this concern entirely, instead pulling Shio in for a commemorative photo and suggesting they exchange contact information. This moment reveals a personality that is straightforward, perceptive, and admirably independent of peer pressure.

Kaoru's primary role in the narrative is to act as a catalyst for Shio's development and a touchstone to her normal life. During the alien attack in Taiwan, Kaoru is separated from her friends and injured, leaving her trapped and in danger. It is the immediate threat to Kaoru's life that triggers Shio's latent powers as an E-Gene holder, allowing her to manifest the weapon known as Nobunagun to save her. After the battle, Kaoru is hospitalized for her injuries. Shio visits her there, feeling overwhelmed by the sudden pressure of being thrust into a world of combat and public attention. Kaoru provides crucial emotional support, thanking Shio for saving her and declaring that they are friends. She encourages a hesitant Shio by assuring her that no matter what she decides or how the world judges her, there will always be at least one person who believes in her. This reassurance is the key factor that convinces Shio to accept her new role and join the organization DOGOO.

The relationship between Kaoru and Shio is the most significant in her story. While Shio initially views Kaoru as untouchably popular, Kaoru sees Shio as someone admirable for being her own person and not conforming to others. After the events in Taiwan, their friendship deepens, and they maintain contact through email, with Kaoru affectionately sending many messages to Shio while she is away on missions. In a later episode, Kaoru is shown visiting Shio in a hospital in Japan, where she once again demonstrates her bravery and quick thinking by helping other children escape from a new monster attack, proving that her courage is not limited to emotional support.

Kaoru undergoes a subtle but meaningful development from a popular schoolgirl into a supportive confidante and a figure of personal strength for the protagonist. Her initial role is as an observer who reaches out to an outsider. After surviving the traumatic alien attack, she becomes the one person Shio can confide in about her fears. Her character growth is less about gaining new powers and more about solidifying her role as an anchor to Shio's humanity, transitioning from a casual classmate to an indispensable friend.

Regarding notable abilities, Kaoru Asao possesses no supernatural powers or combat skills. She is an ordinary human teenager whose primary strengths are her social intelligence, personal bravery in a crisis, and her unwavering, supportive loyalty to her friend, which proves to be a decisive emotional and psychological force in the story.