Jikū Mugenkai is a recurring character from the Jigoku Sensei Nube manga series, known primarily for his role as the father of the protagonist, Meisuke Nueno, also known as Nube. His real name is unknown, and he operates under the alias Mugenkai Jikū, which translates to "Unlimited Space-time," as he works as an exorcist.

In his past, Mugenkai was a kind and compassionate exorcist who performed his services for free, living a happy life with his family. He deeply loved his son, Meisuke, and held him dearly. This changed dramatically after his wife passed away from an illness. The tragedy caused him to see through what he perceived as humanity's true nature, leading him to lose all hope in people. Consumed by this disillusionment, he made the fateful decision to abandon his young son and leave his hometown.

As a result of this emotional trauma, Mugenkai's personality and motivations underwent a complete transformation. He became a cynical and greedy exorcist who is solely motivated by money. This new persona is so extreme that he would even attempt to blackmail anyone who refuses to pay his fees. His abandonment of Meisuke is a key event in his backstory, creating a profound and painful rift between father and son that defines their relationship. After leaving, he never saw his son again.

In terms of abilities, Mugenkai is identified as an exorcist, a profession that involves dealing with spiritual and supernatural threats. While the full scope of his powers is not detailed, his skills are significant enough to operate under a distinct professional alias. His role in the story is primarily tied to his connection with Nube, serving as a figure from the protagonist's past whose actions had a lasting emotional impact.