Kasumi Tomine is one of the three sixth-grade students who make up the Alien Party at her elementary school, tasked with capturing aliens that appear on the grounds. She is often described as a genius-type student and comes from a wealthy family. Kasumi is a prodigy with a wide range of talents, including proficiency in piano, ballet, chess, robotics competitions, and roller skating, all of which contribute to her versatility and effectiveness as an alien fighter.

In stark contrast to her reluctant and fearful teammate Yuri Otani, Kasumi is an enthusiastic and energetic volunteer who joined the Alien Party simply because she likes catching and fighting aliens. Her outward personality is overwhelmingly cheerful, well-mannered, and seemingly perfect, approaching every task with a smile and an incredible drive to be the best. However, this incessant happiness is widely understood to be a carefully constructed mask. Behind this facade lies a deep-seated loneliness and sadness caused by the departure of her beloved older brother, who was sent to a private school abroad. Her fear of being abandoned again makes her react strongly to any suggestion that her new group of friends might break apart.

Kasumi serves as the id or the red oni of the Alien Party trio, acting as a stark contrast to the stoic and thoughtful Kumi Kawamura and the anxious Yuri Otani. Her primary role within the group is that of the ace; her natural talent and positive attitude allow her to handle alien encounters with a level of grace and skill that the others lack. Her symbiotic partner, a Borg that typically attaches to her head, responds to her commands elegantly, enabling her to capture creatures with ease. One of her more unique abilities involves her noticeably thick eyebrows, which contain antennae that focus a sonic attack; the more hair on her head, the more effective this ability becomes.

Kasumi undergoes a significant and traumatic transformation over the course of the story. Her cheerful disposition crumbles when she encounters a powerful alien known as Yellow Knife, who deceives her by taking the form of her absent brother. Desperate to reclaim the lost bond, Kasumi willingly allows this alien to fuse with her body, becoming part Yellow Knife herself. This event fundamentally changes her, giving her heightened senses and alien characteristics. In the manga continuity, which continues beyond the anime, Kumi is reconstructed by her Borg after her apparent death, exploring themes of symbiosis and identity, but Kasumis own fusion remains a critical turning point for her character. Her key relationships are defined by her desperate need to maintain her found family in the Alien Party, fearing the loss she experienced once before.