Kazuma Satou is the main protagonist, a Japanese teenager who was a hikikomori (shut-in) and NEET (Not in Education, Employment, or Training) before his death and subsequent reincarnation in a fantasy world. His life as a recluse began after a traumatic incident in middle school, where he saw his childhood sweetheart, who had promised to marry him, riding on the back of an older boy's bicycle. His death is entirely undignified; believing he was pushing a classmate out of the path of a speeding truck, he actually died from shock and embarrassment after the vehicle turned out to be a slow-moving tractor.

In the parallel world, Kazuma is physically unremarkable, possessing average height with brown hair and green eyes, and typically wears a green adventurer's cape over a simple tunic and pants. His class is "Adventurer," a jack-of-all-trades role that is often looked down upon because it allows him to learn basic skills from other classes but prevents him from mastering any advanced abilities. Statistically, he is average in most areas but boasts above-average intelligence and an abnormally high luck stat.

Kazuma’s personality is a complex mix of cynical pragmatism and reluctant heroism. He is often lazy, greedy, and openly perverted, frequently trying to scheme his way into wealth or leisure without hard work. He has a sharp, sarcastic tongue and is quick to insult or belittle his party members for their antics, which has done little to endear him to the townspeople of Axel. However, this external rudeness masks a deep-seated loyalty. Despite constantly complaining about them, Kazuma genuinely cares for his companions and will go to great lengths to protect them, often putting himself in harm's way. This contradiction has led Megumin to affectionately label him a "tsundere".

His primary motivation shifts over time. Initially, his goal is simply to live a comfortable, lazy life and perhaps find a way to defeat the Demon King so he can return to Japan. However, he quickly becomes entangled in the financial and social chaos caused by his party. As the leader of this dysfunctional group, his role in the story is often that of the exasperated straight man, forced to use his wits to clean up the messes left by his companions while also scheming to pay off their ever-mounting debts. He is forced to take on dangerous quests, not out of a sense of heroic duty, but out of sheer financial necessity.

Kazuma’s relationships are the core of the series. He chose Aqua, the goddess of water, as his "cheat item" out of spite, and their relationship is defined by mutual bickering, insults, and a codependent friendship where neither can succeed without the other but both refuse to admit it. He is the unwilling caretaker of the Archwizard Megumin, who can only cast one massive Explosion spell per day before collapsing, and the Crusader Darkness, a masochistic noble who cannot hit anything but loves being hit. He develops a close bond with Megumin, which evolves into a romantic relationship, built on his respect for her dedication to her unique magic and his willingness to carry her home after she casts it.

While his stats are low, Kazuma’s abilities lie in his cunning and tactical mind. He cannot overpower his enemies, so he relies on underhanded tactics, creativity, and his high luck. He learns a variety of basic skills, such as "Steal" (which he infamously uses to take panties), "Create Water," "Freeze," and "Drain Touch". His true strength is in combining these simple abilities in innovative ways to defeat far more powerful opponents. He also wields a short katana named Chunchunmaru. Over the course of the story, he shows gradual development, moving from a self-serving coward who wants to use his party as shields to someone who takes the lead in dangerous situations and eventually chooses to remain in his new world with his friends rather than return to Japan.