Nasa Yuzaki is the main protagonist of the anime series TONIKAWA: Over The Moon For You. He is an 18-year-old young man who becomes the husband of Tsukasa Tsukuyomi at the very start of the story, and the series follows their life together as a married couple.

Nasa's background is shaped by his unusual name. His father gave him the name Nasa, written with kanji that mean "starry sky," with the hope that he would be as vast and limitless as space itself. However, throughout his childhood, the name made him a target of teasing and mockery. Determined to become someone no one could make fun of, Nasa channeled his frustration into intense self-study. This drive led him to develop exceptional intellectual abilities, allowing him to achieve top marks on national exams and earn a reputation as a prodigy. Instead of pursuing a traditional high school education after being accepted, he chose to work various part-time jobs and eventually became a freelance programmer and consultant, using his skills to improve the businesses he worked for.

Personality-wise, Nasa is serious, logical, pragmatic, and highly organized. He approaches problems with a rational mindset and strives to find the most efficient solution. He is also remarkably calm in the face of disaster, once stating that even if his house burned down, he would be okay because all his important possessions are either digital or replaceable. Despite his sharp intellect in academics and technical matters, he often lacks common sense in everyday social situations, particularly when it comes to romance. He can be flustered easily by his wife's beauty and is frequently awkward in expressing affection, which leads to many endearing and comedic moments. He is also honest, straightforward, and possesses a positive and persistent attitude, getting along well with most people he meets.

Nasa's primary motivation undergoes a significant shift over the course of the story. Initially, he is driven by a desire to make a name for himself and earn respect in order to overcome the humiliation he endured as a child. After meeting Tsukasa, his motivation transforms entirely. His central goal becomes being the best husband he can be for her. He dedicates his life to ensuring her happiness, understanding her, and loving her unconditionally. A secondary and evolving motivation is to unravel the mysteries surrounding Tsukasa's past, particularly the hints of her immortality, and to help her achieve whatever her deepest wish may be.

As the protagonist and narrator, Nasa is the central figure around whom the story revolves. His role is to anchor the series' exploration of marriage, love, and everyday life. He is the one who experiences the joy, confusion, and growth that comes with his sudden marriage. His rational worldview is constantly challenged by the emotional and mysterious nature of his relationship with Tsukasa. He also serves as the primary agent of action, using his intelligence and skills to solve practical problems, such as fixing household appliances, managing finances, and even saving the Arisugawa family bathhouse from financial ruin. Later in the story, his role expands to include investigating the scientific and supernatural aspects of Tsukasa's condition, taking up the research left behind by Tsukasa's guardian.

The most important relationship in Nasa's life is with his wife, Tsukasa Yuzaki. He loves her with complete and unwavering devotion, frequently calling her the cutest girl in the world and going to great lengths to make her happy. His love is immediate and absolute, and he accepts her without question despite her mysterious nature. Their relationship is the emotional core of the series. Nasa also maintains a friendly dynamic with the Arisugawa sisters: Kaname, a perceptive junior high student who often teases him and offers relationship advice, and Aya, his former classmate who initially harbored a crush on him but later becomes a supportive friend. He has a complex relationship with Chitose Kaginoji, Tsukasa's adoptive younger sister, who initially opposes the marriage but eventually comes to respect him. He also develops a bond with Tokiko Tsukuyomi, Tsukasa's elderly guardian, who entrusts him with the truth about Tsukasa's past.

Throughout the series, Nasa undergoes noticeable character development. He begins as a highly pragmatic and goal-oriented loner who meticulously plans his life. Through his marriage, he learns to open up emotionally and embrace a life he never planned. He evolves from someone who relies solely on logic to someone who learns to live in the moment and prioritize his wife's happiness above all else. His intelligence, once aimed solely at academic and professional success, is repurposed to solve everyday domestic problems and to understand the deeper mysteries of Tsukasa's existence. This transition reflects his growth from an ambitious boy into a devoted and caring husband who learns to balance his analytical mind with the unpredictable nature of love.

Nasa possesses no supernatural abilities but is gifted with a high level of intelligence and a wide range of practical skills. Through rigorous self-study, he has gained professional-level proficiency in accounting and computer programming. He is also an adept handyman, able to fix and upgrade household appliances, perform emergency repairs, and solve complex technical problems with ease. His strong organizational skills mean he is always prepared for the worst, keeping backups of important data in cloud storage and insuring his possessions. He is also a capable teacher, able to instruct high school programming classes. Beyond his technical abilities, his defining traits are his unwavering dedication to his wife and his ability to remain calm and rational in even the most chaotic situations.