Kanata Saionji is the main male character in the story. He is a high school student who lives in an old Buddhist temple atop a hill, which is run by his father, Hosho Saionji, a monk. From the beginning, Kanata is established as an exceptional individual, being among the most popular boys at his school due to his good looks, his calm demeanor, and his natural talents. He is a true all-rounder, excelling not only in his academic studies but also in sports, and he carries the responsibility of being the student council president.

In terms of personality, Kanata initially presents a cool and somewhat aloof exterior. He is generally friendly and well-liked, appearing very calm even when faced with dire or unusual situations. However, this surface-level composure masks a more complex inner world, as he is often described as having a sugar-and-ice personality. He has a tendency to deflect and mask his true emotions, particularly those of affection and care, by teasing others. This is most evident in his relationship with Miyu, with whom he frequently argues and bickers, especially in the beginning. Despite this, he is deeply caring and loyal to his loved ones, including his family, his friends, the alien babysitter pet Wannya, and most importantly, the alien baby Ruu and his housemate Miyu.

The central motivation and role for Kanata in the story are forced upon him by circumstance. His father unexpectedly departs for a year-long voyage to India, leaving Kanata alone in the temple. On that same night, a UFO crash-lands at the temple, carrying Ruu, a baby alien from the planet Otto, and Wannya, a shape-shifting alien creature who acts as Ruu's babysitter. Stranded and unable to return home, Wannya asks the two teenagers for shelter, and they agree. This event instantly transforms Kanata from a popular student living alone into an unlikely, makeshift parent. His primary motivation throughout the series becomes the protection and care of Ruu, for whom he feels a deep and genuine love, going to great lengths to ensure the baby's safety and happiness. He and Miyu must also navigate the challenge of keeping Ruu's extraterrestrial origins a secret while pretending to be cousins to avoid the scandal of living together.

Kanata's key relationships form the emotional core of the series. His relationship with Miyu Kouzuki begins with friction, as they are thrown into a sudden living arrangement and must face the chaos of parenthood together. He initially finds it difficult to admit his true feelings for her, often using teasing to hide his growing affection. As the story progresses, their immature bickering gives way to a smooth, gradual development of mutual respect and love, ultimately culminating in their marriage years later. His relationship with Ruu is that of a devoted father, with Ruu coming to see both Miyu and Kanata as his parents due to their similar looks. At school, his calm and attractive nature makes him a chick magnet, attracting the obsessive affection of his classmate Christine, as well as a club of adoring fangirls. He also maintains a friendship with his self-proclaimed rival and fanboy, Santa Kurosu.

Throughout the series, Kanata undergoes significant development. He starts as a somewhat emotionally guarded boy who is forced into an adult situation. Through the shared trials and comedic situations of caring for Ruu, his sense of family deepens, and he learns to embrace his role as a protector and caregiver. The most crucial part of his development is his emotional journey with Miyu, as he slowly stops masking his feelings and moves from denial to openly caring for her, with the two eventually admitting their feelings for each other. While he does not possess any supernatural abilities of his own, his notable abilities are his exceptional intelligence, athleticism, and a remarkable capacity for calmness under pressure, which often proves vital in resolving the various crises the extraterrestrial baby creates.