Izuko serves as the Keeper of the Gate of Grudges, a supernatural guardian responsible for guiding the souls of those who have died unexpectedly, particularly murder victims, at a crossroads between life and the afterlife. Her role is to present each newly arrived soul with three distinct choices. The first is to accept their death and pass through the gate to heaven, where they will await reincarnation. The second is to refuse to accept their demise and remain on earth as a wandering ghost. The third and most difficult option allows the spirit to take vengeance by haunting and killing one living person, but this act comes with the irreversible consequence of condemning their own soul to hell and eternal suffering. Izuko's duty is to explain these options clearly, but a fundamental rule of her position is that she cannot make the choice for any soul; to do so would cause her to fall into hell herself.

The background of Izuko varies across the different adaptations and manga continuities, though several consistent threads explain how she came to hold this position. In the live-action television series and its theatrical prequel, Izuko was originally a young woman named Mina, a bride-to-be who became the victim of a supernatural serial killer who ripped out her heart. After her death, she found herself at the gate where she met the previous Izuko. When that guardian was killed by a demon in a sword duel, Mina inherited the guardian's sword and the title itself. She then confronted and defeated her own demonic killer, an act that permanently cemented her new role as the Gatekeeper. In the original manga, there are alternative origins presented. In one version, Izuko was a stillborn baby who arrived at the gate without even the chance to consider her options, leading her superiors to decide she would serve as the gatekeeper. In another manga storyline, she states that she was aborted, and in a different iteration, she died in a car accident alongside her family after having a prophetic vision of the crash, choosing the role of gatekeeper as a form of repentance.

Regardless of her mortal origins, Izuko is portrayed as a beautiful, calm, and mysterious presence who carries out her solemn duties with deep compassion. Despite her sincere efforts to guide spirits toward a peaceful resolution, she often witnesses tragic and morally complex situations where there are no truly happy endings. Her work brings her into contact with a wide range of souls, each with heartbreaking stories of betrayal, violence, or unfortunate circumstance, and she must maintain a detached yet caring demeanor as she helps them confront the injustices of their deaths. There is a quiet sadness to her character, as she is fully aware that the choices are difficult and that the desire for revenge is powerful, even when it leads to damnation. Her own past as a murder victim gives her a unique and personal understanding of the anger and confusion that the souls feel, though she remains bound by the rules of her position.

Key relationships in her story include the previous gatekeeper who passed on the role to her, as well as other souls she encounters who eventually take on the mantle. One notable figure is Anna, a young woman who was murdered by her own mother and who, after meeting Izuko at the gate and coming to terms with her short and fearful life, decides to become the next gatekeeper herself. This demonstrates that the position of Izuko is not a permanent sentence for one individual but a role that can be passed down, allowing the current guardian to eventually seek peace or rebirth. In the manga, Izuko ultimately decides to take the path toward being reborn, passing her responsibilities on to Anna and then descending to earth to watch over her younger sister one last time before beginning a new life. Across the different versions of the story, Izuko undergoes a form of development from a victim of tragedy into a guide for other victims, and finally into a being who must confront whether she herself can accept the choices she offers to others.

Izuko also possesses several notable supernatural abilities that aid her in her duties. She is often depicted wielding a sword, the weapon of the Gatekeeper, which she inherited from her predecessor and can use to fight demons or other supernatural threats. In some versions of the story, she has the ability to see into the future, a power that can be a source of personal anguish as it was in the incarnation where she foresaw her own family's fatal car accident. Her primary ability, however, is her authority over the gate itself, allowing her to summon or guide souls and to enforce the rules of the afterlife. She can also allow spirits to return temporarily to the world of the living to investigate the circumstances of their deaths or to observe the consequences of their choices, helping them make an informed decision about their final path.