Akira Tsubaki is a seventeen-year-old second-year student at Kazamidai Public High School and serves as the protagonist of the story. He is, by most measures, an ordinary and unremarkable teenager, a point often emphasized by his best friend, Kouhei Ueno, who describes him as someone with average athletic ability and academic performance but a genuinely good person at heart. He lives with his older sister, Youko Tsubaki, who is seven years his senior and took on a maternal role following the death of their mother when Akira was very young. As a result of this early loss, he has few memories of his mother and initially believes he felt no sadness over her passing.

In terms of personality, Tsubaki is characterized by his kind, empathetic, and reliable nature, though he frequently struggles with insecurity and self-doubt, particularly regarding love and his personal relationships. He is known to be a member of the film research club, albeit as a "ghost member" who rarely attends meetings. His only notable talent and source of personal pride is his skill at bowling, which provides him with a rare moment of confidence. While he experiences normal adolescent desires and curiosity about the opposite sex, he shows considerable restraint and is often intimidated by his girlfriend's intense demeanor.

His life takes a dramatic turn when he impulsively tastes a drop of drool left on a desk by the transfer student Mikoto Urabe. This single act creates a powerful physical and emotional bond between them, revealing that he has become addicted to her saliva and will suffer from withdrawal symptoms akin to a fever if he does not regularly ingest it. This strange condition, which Urabe refers to as "love sickness," becomes the foundation of their relationship. Because of this bond, Tsubaki is able to sense Urabe's moods and emotions through her saliva, though the connection is not as refined as her ability to read his thoughts. This daily ritual, which usually takes place after school at a secluded bridge, allows them to communicate on a profound level without always needing words.

Throughout the narrative, Tsubaki's primary role is that of a normal boy navigating an incredibly abnormal relationship. His central motivation is to understand his mysterious girlfriend and to make their relationship feel more conventional and secure, despite its bizarre foundation. He often finds himself trying to bridge the gap between his desire for a typical romantic experience, such as wanting to be called by his first name or trying to hug her, and Urabe's unpredictable and often resistant reactions, which can involve threatening him with the scissors she keeps hidden in her clothing. These challenges serve as a catalyst for his personal growth. Over time, he gradually gains the confidence and wisdom to confront difficult situations and explain his perspective, moving away from simply being an accidental pervert or a flustered boyfriend.

His key relationships shape his development significantly. The bond with Mikoto Urabe is the center of his world, and his interactions with her force him to confront his own feelings and vulnerabilities. For instance, through the telepathic link created by their saliva, Urabe once revealed that he had buried deep sadness over his mother's death, a truth he had forgotten about himself. His friendship with Kouhei Ueno provides a sounding board for his worries, while his history with a middle school crush named Hayakawa introduces moments of jealousy and tests his commitment to Urabe. Despite the bizarre circumstances and the fear of Urabe's sharp scissors, Tsubaki's journey is one of steady devotion, learning to accept the unique terms of their love, and maturing into a reliable partner for his mysterious girlfriend.