Genkai is an elderly martial arts master and one of the most renowned spiritual warriors in the human world. She resides in a secluded mountain temple, having withdrawn from active combat long before the events of the story. In her youth, she was considered a peerless fighter whose power was acknowledged even in the demon realm. Her physical appearance is that of a small, seemingly frail old woman with sharp features, shoulder-length pinkish-gray hair, and a perpetually stern expression. She typically wears traditional martial arts attire along with a purple cap. Despite her aged appearance, she remains a formidable opponent whose reputation commands deep respect from both humans and demons alike.

Genkai possesses a blunt and pragmatic personality that she cultivated through decades of experience, loss, and hardship. She is known for her harsh, no-nonsense teaching style and shows little patience for weakness, foolishness, or those who lack determination. Her stern exterior often comes across as cold, but it conceals a genuine and deep care for those she accepts as her students. She views emotional attachments and sentimentality as potential liabilities, yet these very attachments become central to her character. She is old-fashioned in her values, putting her mind and body through grueling training to master her abilities. One of her unexpected traits is a love of video games, and she owns a private arcade at her compound where she modified some games to evaluate spiritual abilities. She is an accomplished gamer, claiming to have won tournaments and demonstrating remarkable skill during a battle against a powerful opponent who used games as weapons.

Her primary motivation throughout the series is to ensure her powerful techniques do not fall into the wrong hands and to guide her successor away from the mistakes she witnessed in her past. She holds a tournament to find a worthy heir specifically to prevent a demon named Rando from stealing her power. Later, she becomes driven by a desire to prevent her student Yusuke Urameshi from following the tragic path taken by her former partner, the Younger Toguro.

Genkai serves as the primary mentor for Yusuke Urameshi, the series protagonist. She trains him in the fundamentals of spiritual awareness and teaches him her ultimate technique, the Spirit Wave. Her role extends far beyond that of a simple teacher; she becomes a moral anchor and a surrogate grandmother figure. She secretly enters the Dark Tournament disguised as the Masked Fighter to guide and protect Yusuke and his team. Using her wisdom and experience, she helps free enslaved fighters from mind control and navigates the tournament's political dangers.

The most significant relationship in Genkai's past is with the Younger Toguro. They were once fighting partners and implied lovers, an unstoppable team whose partnership ended tragically when Toguro chose to abandon his humanity to become a full demon. Genkai valued her human spirit and identity, and this choice angered her to the point of parting ways with him for decades. Their ideological divide forms the tragic core of her backstory. Her relationship with Yusuke evolves from mutual irritation to a deep, familial love as she comes to view him as the grandson she never had. She also develops a grudging respect for the demons Kurama and Hiei, recognizing their strength while remaining cautious of their natures.

Throughout the series, Genkai undergoes significant development. She begins as a recluse resigned to dying alone with her power untransferred. By taking Yusuke as her student, she re-engages with the world. Her decision to sacrifice her remaining power and ultimately her life to give Yusuke the strength to defeat Toguro represents a profound shift from pragmatic isolation to selfless love. After being resurrected by Yusuke's wish at the end of the Dark Tournament, she becomes more directly involved in the group's struggles, serving as an advisor during subsequent crises. In the final arc, she actively trains other fighters and bequeaths her temple to Yusuke and his friends with the dream that it becomes a sanctuary for peaceful demons in the human world, completing her transformation from a solitary warrior into the matriarch of an unconventional found family.

Genkai possesses a wide array of notable spiritual abilities. Her signature technique is the Spirit Wave, a fighting style that channels spiritual energy through the entire body, effectively turning the user into a living weapon. When she exerts her full power, her cells revitalize and she temporarily reverts to the physical prime of her early twenties. She is capable of firing a Spirit Gun from her finger, as well as a Spirit Shotgun that scatters multiple smaller blasts over a wide area. The Spirit Wave Orb is a concentrated sphere of her life force that she can transfer to another person to vastly increase their power, though the process is incredibly dangerous. She also masters the Spirit Reflection Blast, a counter technique that absorbs an opponent's energy and redirects it. Beyond combat, her wisdom extends to advanced spiritual abilities including healing, the power to free minds from control or possession through purification, creating barriers, and performing spiritual surgery to return displaced souls to their bodies. Even after giving away the majority of her power, she remains one of the most dangerous and respected fighters in the human world.