Kaiji Itō is the central protagonist of the story, a young man whose life is defined by debt, gambling, and an uncanny ability to thrive only when pushed to the absolute brink of disaster. He is introduced as a dejected and shiftless individual, a resident of Tokyo's lower depths who struggles to find or maintain steady employment. His days are spent drinking inexpensive alcohol and indulging in small-time gambling, often losing the little money he possesses. He is lazy, self-indulgent, and lacks any real ambition for the future, leading a powerful figure in his life to label him as a form of human scum. This apathy stems from a deep-seated resentment of a society where he feels he has no chance to succeed, and he occasionally vents his frustration by vandalizing the expensive cars of the wealthy. At the start of his story, he is burdened with a massive debt of nearly four million yen after naively cosigning a loan for a coworker who then disappeared, leaving Kaiji to face the full, compounded consequence.

Kaiji's personality is a complex and often contradictory mix of cowardice, impulsive rage, deep-seated compassion, and brilliant cunning. In ordinary, peaceful circumstances, he is prone to whining, procrastination, and a general lack of motivation. He can be hotheaded, easily angered by perceived slights or accusations, and often makes irrational decisions, particularly when it comes to his gambling addiction. He seems to believe in a gambler's fallacy, frequently choosing to risk his winnings again because of a past lucky result, a flaw that consistently lands him back in trouble. However, once placed in an extreme, life-threatening situation, his true nature emerges. His mind becomes razor-sharp, focused, and incredibly logical. He displays extraordinary courage, resourcefulness, and an almost superhuman will to survive. Despite the desperate circumstances, he retains a strong moral compass and values human life greatly. He is often compelled to help those who are downtrodden and cornered, even if it means sacrificing his own chance at profit or safety. This compassion frequently leads to him being betrayed by those he trusts, which in turn makes him bitter and causes him to construct a harsh, cold-hearted facade to protect himself, a lie that he can never truly maintain for long.

His primary motivation is survival and a desperate desire to escape the crushing cycle of debt imposed by the Teiai Corporation. However, this desire for freedom is constantly at war with a deep-seated craving for the adrenaline and purpose that only high-stakes gambling provides. He admits that he cannot take meaningful action in his life unless he is under extreme conditions. After each victory, when he has a chance to live peacefully, he quickly reverts to his lazy, self-indulgent ways, eventually finding himself bored and drawn back to the very world of dangerous gambles that nearly destroyed him. This self-destructive impulse is fueled by a growing, almost obsessive hatred for the powerful figures who run Teiai, and a desire to not just win money, but to humiliate them and prove their cruelty is not absolute.

In the narrative, Kaiji serves as the unlikely hero who repeatedly challenges the oppressive system of Teiai. His role is not that of a flawless genius, but of a desperate everyman forced to become a legend. He is the one who rallies other desperate debtors, forming them into teams like the 45ers, and devises ingenious strategies to overcome seemingly impossible games rigged against him. His key relationships are often defined by betrayal and a painful, recurring cycle of trust. He is constantly taken advantage of by fair-weather allies who abandon him when he is most vulnerable. Despite this, he forms genuine bonds with a few individuals, such as the older gambler Kōtarō Sakazaki and his later companions, Chang and Mario, for whom he is willing to risk everything. His central antagonistic relationships are with the various executives of Teiai, men who view him as nothing more than a plaything, yet are repeatedly shocked by his resilience and cunning.

Throughout his journey, Kaiji undergoes a significant but cyclical development. He starts as a naive and trusting man, but after being repeatedly betrayed, he becomes more calculating and cautious. Each gambling arc forces him to grow, pushing him to new heights of strategic thinking and psychological insight. He learns to see through simple deceptions, to manipulate probabilities, and to read his opponents' intentions. Yet, his core character flaw remains: he cannot help but gamble. Even after winning vast sums of money and securing his freedom, he chooses to risk it all again, ultimately losing everything and returning to a state of debt and despair. His physical appearance changes to reflect his ordeals, gaining a scar on his cheek, scars on his left hand from reattached fingers, and a reattached left ear, all permanent reminders of the price he has paid.

Kaiji possesses no supernatural abilities. His notable abilities are entirely mental and emotional. He is a genius of logical deduction under pressure, able to dissect complex, multi-layered games to find a fundamental weakness. He is highly skilled at reading people, being able to detect lies, fear, and subtle changes in behavior that signal an opponent's strategy. He is also a charismatic leader in a crisis, capable of inspiring other terrified men to band together and take collective action against their oppressors. His greatest ability, however, is his sheer tenacity; no matter how hopeless the situation, how terrible the betrayal, or how severe the physical and financial loss, Kaiji always finds a way to stand back up and continue fighting. He is a brilliant fool, a man who can only truly shine when the lights are about to go out forever.