Genjūrō Tsukishima is the father of Jinbei Tsukishima, the main protagonist of the series, and a figure of immense, albeit compromised, strength. He is a master swordsman of renowned skill who once operated as a member of a criminal organization known as the Hunter Eight, which had attempted to assassinate a magistrate. This past, however, is overshadowed by a defining event that shapes his relationship with his son.

Personality-wise, Genjūrō is depicted as a strict and demanding individual. His primary drive is the relentless pursuit of greater strength, a goal he pursues with single-minded dedication. This intense focus defines his character and his approach to life, particularly his role as a father and teacher.

His primary motivation stems from a past failure involving his young son. During a mission to escort a lord, Jinbei failed to protect the lord's son, who was so frightened by a giant insect that he wet himself. To spare Jinbei from being imprisoned for this dishonor, Genjūrō took full responsibility. His act of atonement was brutal and absolute: he deliberately severed the ligaments in his own left leg, permanently crippling himself. This sacrifice ended his own path as a warrior but was made to secure his son's future. Consequently, a major part of his motivation becomes channeling his ambitions through Jinbei, sending him to Edo to join the Insect Magistrate organization in his place to atone for the past incident and continue the family's legacy of strength.

In the story, Genjūrō plays a crucial role as the origin of the protagonist's journey. He is the first choice of Kotori Matsunohara to be a member of the Mushibugyo, but due to his broken leg, he is unable to accept. Instead, he nominates his son, setting the entire plot in motion. He remains a powerful off-screen presence, representing the level of strength that Jinbei aspires to reach.

Key relationships define his past and present. His bond with his son, Jinbei, is complex and expressed through rigorous training and physical conflict, as they show affection by fighting each other. Genjūrō has trained Jinbei since childhood, instilling in him the basics of his sword style and the mindset of a warrior. His relationship with his wife, Kanae, is also significant. They met in Edo while Genjūrō was traveling to perfect his sword style. Their courtship was unusual, consisting of duels that lasted an entire year, during which they fell in love and eventually married. Kanae is noted as being one of the few people who could match his strength.

While Genjūrō's personal development largely occurs before the story begins, culminating in his leg injury and a shift from active combatant to a mentor figure, his ideology and techniques are developed through his son. His most notable development is the creation of his own unique sword style. He is a very insightful and resourceful man who single-handedly designed this style, which includes a basic set of techniques and a deeper repertoire of thirty-six secret sword techniques.

Regarding notable abilities, despite the severe handicap of his broken left leg, Genjūrō remains an immensely powerful combatant. His skill with a blade is such that he can single-handedly slay dozens of giant insects in a matter of moments. His true power, however, lies in his intellectual contribution to swordsmanship: the Tsukishima style he created. This legacy of techniques is what he imparted to Jinbei, forming the foundation of his son's fighting ability and serving as Genjūrō's lasting impact on the world of the series.