Endo is a debt collector and the president of a loan company affiliated with the Teiai Group, the powerful financial empire that controls much of the criminal underworld in the series. He is a sleek, calculating figure who operates as a cog in a much larger machine of exploitation. Endo first appears at the protagonist Kaiji Ito's apartment, wielding an old co-signed loan to ensnare Kaiji in a life of high-stakes gambling. Presenting himself as a facilitator offering a way out, he pressures Kaiji into boarding the gambling ship Espoir, a decision that sets the entire story in motion.

Endo's personality is defined by a chilling blend of sinister cunning and pragmatic self-interest. He is a bloodless moneylender to his core, possessing a thorough understanding of a debtor's psychology and skillfully manipulating them with convincing rhetoric. His outward demeanor can be leisurely, even friendly, but this masks a ruthless interior that will not hesitate to commit acts close to fraud or outright betrayal for personal gain. Endo views the world through a purely transactional lens, believing that society is a brutal, eat-or-be-eaten environment where sentimentality and trust are fatal weaknesses. He is fundamentally cynical, showing disappointment in Kaiji's persistent kindness and seeing it as a naive inability to understand the true nature of the world.

Initially, Endo's motivations are purely professional: he serves as a recruiter and agent for Teiai, earning his living by driving desperate people further into debt. However, his goals become more personal after suffering a significant reversal of fortune. As a former subordinate of the Teiai Group's second-in-command, Yukio Tonegawa, Endo enjoyed a comfortable and promising position within the organization. When Kaiji defeats Tonegawa in a gamble called the E-Card, it leads to Tonegawa's downfall, and Endo's own status collapses with him. He is relegated to menial debt-collection duties, a humbling fall from grace that fuels his resentment and his desperation to reclaim his former standing.

In the story, Endo serves as a recurring antagonist and a dark mirror to the protagonist. His role evolves from a tempter who initiates Kaiji's journey to an active, treacherous partner. He first sets Kaiji on the path of desperation and, after Kaiji survives the Espoir, betrays him again by sending him to an underground labor camp. The most complex phase of their relationship occurs during the "Swamp" arc, where Kaiji, needing financial backing and manpower, is forced to ally with Endo. Despite their shared enmity, they cooperate to rig a pachinko parlor. Endo provides the funding and a set of hundreds of pre-made印章 (hanko stamps) to forge guarantees, showcasing his meticulous and unscrupulous nature. This alliance is a marriage of convenience built on mutual necessity, not trust.

Endo's relationship with Kaiji is the most pivotal in his arc. He views Kaiji with a mixture of contempt and grudging respect for his survival instincts and genius. While he consistently tries to exploit and cheat Kaiji, there are moments that suggest a conflicted professional admiration. However, Endo ultimately adheres to his cynical worldview. In the climax of the Swamp arc, after Kaiji wins a fortune, Endo drugged him and Sakazaki, then revealed a hidden clause in a contract Kaiji had signed in haste. The clause applied a predatory interest rate of thirty percent compound interest every ten minutes, allowing Endo to legally abscond with nearly all of Kaiji's winnings. This final, devastating betrayal was a calculated act. Endo later offered a degree of self-justification, explaining that Kaiji's unwavering, almost foolish kindness would always lead him to ruin. In Endo's view, taking the money was inevitable, a harsh lesson for a man too good for a predatory society.

Despite his treachery, Endo demonstrates notable abilities. He is a master of contract law and loopholes, weaponizing fine print with devastating effect. He is also a skilled strategist and financier, capable of organizing complex operations like the Swamp raid. His primary ability, however, lies in psychological manipulation; he can read a target's desperation and accurately predict their actions, using this insight to spring traps like the one on the Espoir or the labor camp ploy. Through the course of the series, Endo undergoes a form of development as he is humbled by his fall from grace and then claws his way back to financial security by betraying Kaiji. He ends the arc having paid off his own debts to Teiai, a testament to his survival skills, but he remains a fundamentally unchanged predator, leaving Kaiji to face his next struggle alone and penniless.