Genkai is a pivotal human character who serves as a mentor and master of spiritual arts. At first glance, she appears as a small, elderly woman with light, wavy pinkish-gray hair and brown eyes, typically dressed in martial arts clothing featuring a long red sleeveless tunic with green trim. Her appearance is deceiving, as she is one of the most powerful human fighters in the world. When she fully exerts her spiritual energy, her cells temporarily revert to the state they were in at the peak of her life, transforming her into a young woman with vibrant pink hair and considerably greater strength.

Her personality is shaped by a lifetime of rigorous training and immense power. She is an old-fashioned, pragmatic woman with a cold and hard demeanor on the surface, speaking sharply and often berating her students. This toughness masks a deep-seated determination and a strong moral core. She is willing to do almost anything she deems necessary to accomplish her goals, but she is not without decency or remorse, as seen when she intervenes to prevent a needless death during a tournament match. Genkai has a notable love for video games, even owning a private arcade at her mountain compound and proving herself highly skilled at them.

Genkai's motivations are rooted in a desire to pass on her knowledge and ensure her powerful techniques do not fall into the wrong hands. She is driven to find a worthy successor who possesses both strength and the right character. A significant part of her past involves a close partnership with the warrior Younger Toguro, with whom she once won the Dark Tournament. She became disillusioned when he wished to become a demon for power, and she carries the burden of that history, determined to prevent others, especially Yusuke, from making the same mistake.

In the story, Genkai first holds a tournament to determine her successor, where Yusuke Urameshi ultimately becomes her student. She puts him through grueling, life-threatening training to unlock his potential. During the Dark Tournament, she disguises herself as a masked fighter to join Yusuke's team and guide him from within. She ultimately transfers the bulk of her spiritual energy, the Spirit Wave Orb, to Yusuke to give him a chance against Toguro. After being killed by Toguro, she is later resurrected by the wish of Team Urameshi for winning the tournament. In subsequent arcs, she continues to act as a strategic advisor and mentor, training new fighters and helping her students navigate complex spiritual and personal challenges.

Key relationships define her role. Her bond with Yusuke Urameshi is central; she initially views him as a slacker but soon comes to love him like a grandson. Their relationship is tempestuous and filled with trash talk, but built on profound mutual respect. Her history with Younger Toguro is another crucial relationship. They were once partners and possibly lovers, but his pursuit of demonic power turned him into her bitter enemy. Their final confrontation is a tragic and defining moment for both characters.

Throughout the series, Genkai demonstrates a range of notable spiritual abilities. The Spirit Wave is her ultimate fighting style and technique, which uses the body as a center for spirit energy and can restore her to her youthful prime. She is also skilled in the Spirit Gun, firing a massive, devastating blast. The Spirit Wave Orb is the compressed bulk of her power, which she transfers to Yusuke. The Spirit Reflection Blast allows her to absorb an opponent's energy and redirect it against them. She also uses techniques like the Spirit Shotgun for wide-area attacks, the Fist of Cleansing to purify evil influences, and the Spirit Cuffs to restrict a student's energy as a form of intense training.