Father Nitani is a character from the later arcs of the story, distinguished by an unusual and complex background that sets him apart from many other characters. Before becoming a man of faith, he was a member of the yakuza, the Japanese criminal underworld. His life took a profound turn during a period when he was on the run in China. It was there that he encountered a Catholic priest who needed to deliver a truck full of medicine to a remote village during a heavy storm but could not drive the vehicle. Nitani agreed to help, though his initial motivation was merely to get away from the police who were pursuing him.

During this harrowing journey, the priest demonstrated an extraordinary and unwavering faith in God. At one point, the truck needed to cross a flooded river with a bridge too damaged to bear its weight. The priest got out of the truck to steady a beam, allowing Nitani to drive across. After crossing, Nitani looked back and saw the priest swept away by the raging water, seemingly killed by the very faith he held so dear. Grief-stricken, Nitani completed the journey alone, delivered the medicine, and was hailed as a hero by the villagers. Overcome with despair for the seemingly foolish and lost priest, he began to weep. Only then did he see the priest, alive, limping towards him. This transformative experience directly led to his vocation. The priest he helped that day would eventually become the Pope. This deep, personal connection to the Pontiff forms the core of his motivational drive.

Personality-wise, Father Nitani is defined by the unique blend of his past and present. Having lived as an outlaw and witnessed what he considers a miracle of faith, he possesses a certain detachment from the trivial concerns of everyday life, a man who has seen both the worst of humanity and the heights of sacrifice. He is pragmatic and resilient, qualities likely carried over from his days as a yakuza, but they are now channeled into his priestly duties. He serves as a priest in the Kabukicho district of Tokyo, a notorious red-light district, suggesting a calling to minister to those on the margins of society, people he might have once called his peers.

His primary role in the story unfolds when the global threat orchestrated by the Friend escalates. As the Friend's influence grows, the Pope in Rome becomes a prisoner in the Vatican, isolated and in danger. Father Nitani is summoned by the Pontiff, and he becomes a crucial agent attempting to reach and protect his old friend and spiritual mentor. His key relationship is, therefore, directly with the Pope. He arrives in Rome and is determined to gain an audience, but finds himself unable to enter the Vatican due to the chaotic state of the world. He is even turned away because he cannot prove he is not a space alien, a concept propagated by the Friend's followers. This obstacle does not deter him. He shows patience and resourcefulness, waiting near the Vatican day after day. He eventually makes contact with a young boy and his family, who are trusted cooks for the Pope, and through them, he is able to receive a clandestine message from the Pontiff himself. In one instance, he manages to warn the Pope that he is about to be replaced by an imposter loyal to the Friend.

His development is largely a retrospective one, revealed through his powerful backstory that explains his transformation from a fugitive yakuza into a priest. In the present timeline, he is a fully realized character of unwavering conviction. While he does not possess any superhuman or martial arts abilities, his notable skills include a remarkable resilience and an ability to navigate dangerous situations, a remnant of his former life. His greatest strength is his profound faith, not one that is naive or untested, but one forged in the crucible of witnessing a miracle after the deepest despair. This makes him a quietly heroic figure, a man whose past has prepared him for the extraordinary task of trying to save the spiritual leader of a world falling into chaos.