Yusuke Fujisaki, known almost exclusively by his nickname Bossun, is the founder and leader of the Sket-dan at Kaimei High School. He is a seventeen-year-old student with black, somewhat curly hair that sticks up on the sides, light brown eyes, and stands at 171 centimeters tall. His signature appearance is defined by the red cap with cat-like horns, a gift from his friend Himeko, and the pair of goggles he wears over it, which originally came from another friend, Switch. He usually wears a customized long-sleeved shirt with the word "SKET" on the front.

Bossun's personality is a blend of energetic enthusiasm and sharp intelligence, wrapped in a somewhat childish and socially awkward exterior. At his core, he is a kind and idealistic individual who possesses a powerful, almost compulsive need to help anyone in trouble, a trait he inherited from his biological parents. He approaches each request with a sense of adventure and unwavering dedication, refusing to give up easily even when faced with difficult odds. Despite this heroism, he is not a stoic or fearless leader; he complains frequently, can be lazy, and often acts in a goofy or immature manner, such as playing with toys or getting dragged into comedic arguments. He is notably awkward around attractive girls, becoming flustered and unable to act normally, which is a frequent source of humor. While he is the protagonist, he is often the target of jokes about being generic or useless for a main character, and he is keenly aware of this, sometimes slipping into a "submissive mode" when he feels overshadowed or unnecessary. He is also known for breaking the fourth wall, commenting on his own lack of screen time or defending his position as the lead character.

His primary motivation stems from a deeply personal place. As the head of the Sket-dan, a club dedicated to solving problems for other students, his driving force is simply to be of service, but the origin of this drive is tragic. He was raised by a woman, Akane, believing her to be his biological mother, along with a younger sister, Rumi. On his fifteenth birthday, he discovered the truth: his birth parents, Ryousuke and Haru, were close friends of Akane who died in two separate accidents on the day he was born, with his father having died saving a child and his mother perishing in childbirth. Shocked and feeling completely alone in the world, he ran away from home. After a period of wandering, he encountered a boy who had been saved by his father and read letters left by his biological parents expressing their hope that he would grow up to help others. Though initially angry, he ultimately realized he could not deny his inherited nature, returning to his adoptive family with a newfound resolve to live a life of service. This desire to help is not for recognition but because he genuinely cannot stand by and watch someone suffer.

In his role within the story, Bossun acts as the strategist and leader of the Sket-dan. While his two teammates, Himeko and Switch, provide physical strength and technical expertise respectively, Bossun is the heart and mind of the operation. He is frequently the one who analyzes the core of a problem, devises a plan, or uses his unique talents to find a solution. His leadership is less about giving orders and more about inspiring his friends and clients through his earnestness and unwavering commitment to justice.

Key relationships define his character significantly. His bond with Himeko and Switch is the foundation of the series. They are his closest friends and confidants, and he is fiercely protective of them; seeing someone insult them or bring up their painful pasts is one of the few things that can make him genuinely angry. The most important relationship in his development is with Sasuke Tsubaki, the strict vice-president of the student council and his self-proclaimed rival. Their constant bickering and competitive dynamic take a dramatic turn when it is revealed on Bossun's seventeenth birthday that they are, in fact, long-lost fraternal twins, having been separated at birth and adopted by different families. This discovery profoundly affects Bossun, who had believed he had no living blood relatives. The revelation brings him to tears, offering a sense of connection he thought he had lost forever. Though they continue to argue and compete, their relationship deepens into a genuine, if awkward, brotherly bond.

Bossun undergoes significant emotional development primarily through this revelation about his origins. He begins the story believing he is alone in the world aside from his adoptive family, having made peace with that fact. Learning he has a twin brother shatters and then reaffirms his understanding of family. He is also shown to mature as he approaches graduation, demonstrating a forward-thinking and selfless nature by deciding to postpone college in order to travel the world and learn from a volunteer he met, intending to dedicate his life to helping others on a global scale.

Despite his sometimes lazy demeanor, Bossun possesses several notable abilities. His most distinctive skill is his "Concentration Mode," activated by pulling down his goggles and focusing his mind. In this state, he can recall minuscule details, filter out all distractions, and perform stunning feats of deductive reasoning and analysis, though it is physically exhausting, often leaving him coughing and drained. Under this focus, he becomes an expert marksman with his weapon of choice, a slingshot that fires metal balls with incredible accuracy. Beyond this, he displays a remarkable and versatile artistic talent, being able to draw, create lifelike origami, and assemble models with expert-level skill very quickly. He is also exceptionally good at solving puzzles and quizzes, making him the "god" of riddles to his less patient teammate, Himeko.