Kei Kusanagi is the central protagonist of the story. He is a young high school student living in a quiet rural town with his aunt and uncle, who serve as his guardians. While he appears to be a fifteen-year-old boy, his actual age is eighteen. This discrepancy is a result of a rare and unexplained condition that causes him to enter a comatose-like state known as a standstill. During these episodes, which are triggered by extreme emotional distress, his body does not age. One such standstill lasted for three years, effectively freezing his physical development and leaving him with the appearance of a younger teenager. Physically, Kei is characterized by his tan skin, cut light brown hair, and light brown eyes, and he is almost always seen wearing glasses.

In terms of personality, Kei is initially presented as a shy, frail, and somewhat reserved individual. He tends to avoid asserting himself and is cautious in his interactions with others, a trait that is largely a consequence of his medical condition. Because emotional upheaval can trigger a standstill, he has learned to suppress his feelings to avoid falling into another stasis. This has led him to become a kind-hearted but carefully controlled person, often described as a nice guy who cares deeply for his family and, later, is very protective of the people he loves. Despite his quiet demeanor, he is intelligent and perceptive, though his primary motivation throughout the story is to navigate the complexities of his secret life while maintaining normal relationships.

His ordinary life is turned upside down following a chance encounter. While sitting by a lake one night, Kei witnesses a UFO landing and sees a woman emerge from the water. To his shock, this woman, Mizuho Kazami, appears the next day as his new homeroom teacher and moves into the house next door. Kei quickly discovers that Mizuho is an alien who has been sent to Earth to observe humanity. Mizuho is desperate to protect her secret identity, and Kei, bound by his own secret regarding his age and condition, becomes her reluctant confidant. Their relationship escalates dramatically when they are discovered together in a locked storage room by Kei's uncle and the school principal. In a desperate attempt to explain the compromising situation, Kei's uncle impulsively claims the two are married. To avoid further scrutiny and protect Mizuho's secret, the pair officially, though secretly, complete the marriage paperwork, forcing them into a fabricated domestic life that neither anticipated.

Throughout the series, Kei's role is to balance his life as a student with his new, secret identity as the husband of his teacher. His primary relationships are central to his development. His dynamic with Mizuho evolves from one of nervousness and obligation to genuine affection and love. He becomes a source of stability for her as she learns about Earth, and she in turn helps him confront his emotional isolation. Key relationships with his classmates also shape his journey. He shares a special bond with Ichigo Morino, a friend who suffers from the same standstill illness, making her one of the few people who truly understands his struggles. Another classmate, Koishi Herikawa, develops feelings for him, creating a complicated romantic subplot that forces Kei to confront his own emotions and his commitment to Mizuho. His uncle and aunt provide a stable home environment, and their house serves as a frequent setting for the unfolding drama.

Kei undergoes significant development from a passive, detached observer of his own life to an active participant. Initially resigned to hiding his illness and avoiding strong feelings, his forced proximity to Mizuho compels him to engage with his emotions rather than suppress them. He learns to stand up for his relationship and himself, transitioning from a boy who could not afford to feel to a young man who embraces his love for Mizuho despite the risks. A notable aspect of his character is a deep-seated trauma involving his older sister, a tragedy that is implied to be the root cause of his initial standstill and his fear of emotional attachment. In the sequel series, Please Twins, he appears as an older student (twenty years old, though his body appears seventeen), where his marriage to Mizuho has become common knowledge among his peers, indicating a long-term resolution to the central conflict of his secrecy.

Regarding notable abilities, Kei does not possess any superhuman powers or specialized skills. His only defining medical trait is his susceptibility to standstills, which is portrayed as a weakness and a significant obstacle rather than a beneficial ability. His true strengths lie in his inherent kindness, his growing resolve, and his loyalty to those he cares about.