Kei Oumi is a student at the Tokyo Space School and a close friend of the protagonist, Asumi Kamogawa. She is characterized by her outgoing, energetic, and friendly nature, often described as a perky loudmouth with a go-getter attitude. Her motivation for pursuing a career in space exploration comes from being inspired by the pictures of the first lunar landing taken by Neil Armstrong. Physically, Kei is an adolescent girl with brown hair that reaches her shoulders, which she often wears in a ponytail, and blue eyes. She is sometimes depicted with a mole and a bow tie.

From her first appearance, Kei is portrayed as someone who is very open and makes friends quickly. She meets Asumi during the difficult entrance examination for the space school and becomes an instant and loyal friend, even as her forthright nature is seen by some as emotionally tone-deaf. Throughout the story, her role is that of a supportive and encouraging friend, but she is not without her flaws. She can be impatient and demonstrates feelings of jealousy, particularly towards their other classmate, Marika Ukita. Kei is seemingly accustomed to being the odd one out in social situations, and this rivalry adds a layer of tension to the group dynamic.

As the narrative progresses, Kei’s character deepens beyond that of a simple, cheerful supporter. She shows significant development when she decides to take on a more active role in helping Asumi cope with a tragedy. When the weight of the past becomes too much for Asumi to bear, Kei leads a personal campaign to lift her friend’s spirits and relieve some of the burden she has been carrying. This action transforms her from a rival for attention into a proactive force for healing within the group. Her arc also includes a subtle change in appearance; she initially wore glasses but later switched to contact lenses.