Atsushi Ohtani is one of the two central characters in the story, a high school boy from the Osaka Prefecture whose most defining physical trait is his remarkably short stature. Standing at approximately 156 centimeters, or about five feet one inch tall, he is frequently mistaken for an elementary school student, a source of great frustration for him. His appearance is youthful, with short hair, which in the anime adaptation is a light red color, and he tends to dress in fashionable casual clothes when not in his school uniform. Despite his lack of height, he has a lean, athletic build.

Ohtani's personality is a complex mix of bravado and genuine kindness. He often acts in a proud and arrogant manner, a defensive mechanism that masks his deep-seated insecurities about his height. This sensitivity was exacerbated by a past relationship in junior high with a girl named Mayu Kanzaki, who broke up with him for a much taller boy. He can be thoughtless and rude, especially in his constant verbal sparring matches with his classmate, Risa Koizumi, with whom he shares a talent for comedic banter. Underneath this prickly exterior, however, he is a very kind and funny person, possessing a gentleman-like instinct to cheer up women who are upset. He is notably not very intelligent and is exceptionally naive when it comes to matters of romance, requiring multiple direct confessions and the intervention of friends before he can comprehend someone's romantic feelings for him.

Ohtani's primary motivation is his love for basketball. Despite the obvious disadvantage of his short stature, he is an aspiring and talented player who becomes a starter and later the captain of the school's basketball team. His passion for the sport is so profound that it shapes his future goals; he studies intensely, despite his academic struggles, to gain admission to a university with the dream of becoming a teacher so he can coach young children.

In the story, Ohtani serves as the reluctant partner in a unique duo with Risa Koizumi. They are initially placed in the same class, and their contrasting heights—she is exceptionally tall—make them a comedic pair often referred to by the nickname All Hanshin-Kyojin, a reference to a famous comedic duo. Their relationship evolves from bickering enemies to friends who agree to help each other pursue other crushes. This plan fails, and Ohtani eventually becomes the object of Risa's persistent affection, a notion he is initially unable to accept. His role is that of the stubborn, often clueless, but ultimately sincere love interest whose emotional journey toward accepting his own feelings forms the backbone of the narrative.

His key relationships are primarily centered on Risa. Their dynamic is characterized by loud arguments, easygoing fun at arcades or concerts, and a deep, unspoken bond that grows over time. He is fiercely protective of her, displaying an extremely jealous side when other men, such as a teacher named Maity or a co-worker named Kohori, show her attention. Other significant relationships include his brief crush on Risa's friend, Chiharu Tanaka, which ends when he learns she does not see him as a romantic prospect, and his ex-girlfriend Mayu, whose rejection continues to haunt him.

Throughout the series, Ohtani undergoes notable development. He transitions from a boy who cannot conceive of Risa as a girlfriend to someone who confesses his love for her, though he remains easily embarrassed by public displays of affection. He faces relationship challenges, including a period of separation driven by jealousy and misunderstanding, which ultimately reinforces his commitment to Risa. His journey also includes an academic one, as he overcomes his reputation as a poor student to achieve his goal of entering university, demonstrating significant perseverance. His notable abilities, beyond his skill on the basketball court, include a surprising talent for motivating others and an affinity for dogs, which allows him to connect even with the most intimidating canines.