Asumi Kamogawa is the central character of the story. Her life has been profoundly shaped by a tragic event that occurred when she was only one year old: Japan's first manned rocket launch ended in a catastrophic failure, and the rocket, named the Lion, crashed into the coastal city of Yuigahama. Asumi survived the disaster because her mother, Kyouko, protected her, but Kyouko was left with severe burns and fell into a coma from which she never woke, dying five years later. Following this loss, Asumi was raised by her father in a home marked by both love and financial struggle.

Shortly after her mother's funeral, a six-year-old Asumi encountered a mysterious figure she calls Mr. Lion. He appears to her as a man wearing a lion mascot's head, and he is the ghost of the pilot who died in the rocket crash that orphaned her. Asumi is the only person who can see or hear him, and he becomes her constant companion, mentor, and a representation of the piece of her life lost in the disaster. It is through her conversations with Mr. Lion, who teaches her about the cosmos, that her childhood fascination with space solidifies into a powerful and unwavering dream: to become an astronaut, or as she puts it, a driver on a rocket.

Asumi's personality is defined by a remarkable combination of gentle kindness and fierce resilience. Despite the trauma of her past, she is an optimistic and resourceful girl who faces the difficult path to becoming an astronaut with determination. Her kindness is a defining trait, as she is consistently loyal to her friends and willing to put herself at risk to help them. During her entrance exam for the Tokyo Space School, her priority is to build mutual respect and friendship with her project partners, sometimes valuing that bond even above passing the test itself. She is also known for being open and honest with her feelings, a quality that allows her to connect with others, including the initially cold and distant Marika Ukita. However, her positive attitude is not a sign of naivety; she is a deeply complex and damaged person. Her ongoing need for Mr. Lion's counsel reveals the psychological wounds she carries, and she is acutely aware of the fragility of life, a lesson learned from her mother's death.

At the start of the main story, Asumi is thirteen years old and small for her age, a physical trait that often leads others to underestimate her and serves as an additional hurdle in her training. Her small stature means she requires a custom-made pressure suit, which creates financial and bureaucratic obstacles for her family and the school. She also has a fear of water stemming from a near-drowning incident in her childhood, which becomes a significant challenge during the space program's physical training, which includes swimming exercises in full gear. Her family lives in perpetual poverty, a fact that surprises her wealthier classmates, and her father works as a laborer, having once been employed by the consortium that built the rocket that caused the disaster.

Asumi's primary motivation is her dream of going to space, a goal she has held since she was a small child. She enrolls in the astronaut training course at the Tokyo Space Academy, a newly established and prestigious school, to pursue this ambition. Her dream is not merely about personal achievement; it is deeply intertwined with her relationships with the dead. She wishes to go to space to be closer to the stars, which she was told are where dead people go, and to honor the memory of her mother and the astronaut Mr. Lion.

Within the story, Asumi acts as the emotional core of her group of friends, which includes her childhood acquaintance Shinnosuke Fuchuuya, the outgoing Kei Oumi, the brilliant but reserved Marika Ukita, and the cool-headed Shu Suzuki. Her relationship with her father is central to her personal life. He is a grieving, hardworking man whose own past is tied to the space program, and he initially struggles to support her dangerous dream, partly due to his own trauma and partly due to a hidden history with a professor at the academy.

Throughout her time at the academy, Asumi develops significantly. She confronts her physical limitations and her fear of water, endures grueling survival training, and faces discrimination from a professor who has a grudge against her father. She also experiences profound grief, such as when her friend Shu dies from a hidden illness, an event that forces her to mature and process loss in a new way. Despite these setbacks, her spirit and dedication never waver. Her development shows her transitioning from a child clinging to a dream born of trauma to a resilient young woman who works with disciplined effort to make that dream a reality, ultimately being the only one from her original group to pass the final exam.

Her most notable ability is her unique perception of Mr. Lion, which provides her with guidance and emotional support that others lack. Beyond this supernatural connection, her true strengths are her emotional intelligence, her unwavering loyalty, and her ability to persevere through any difficulty with a quiet but unbreakable resolve.