Tsukumojuku becomes George's best friend and the pair solves several detective cases together. After an accident in 1904 that causes him to travel through time into the 37th universe in 2012, he eventually helps both protagonists as a spirit.
Although Tsukumojuku is never drawn and his appearance is left to the interpretation of the reader, he is described as the most handsome boy in George Joestar's class. He gives off an aura that causes anyone to pay attention to him when he walks into a room or speaks. In his own novel, his parents considered abandoning him because he had a "birth defect of being too beautiful".
Despite being a young boy, Tsukumojuku speaks in a mature, well-mannered fashion. George states that he is the most intelligent yet also most bizarre boy in their class. As a child detective, Tsukumojuku lacks modesty and is desensitized to gruesome murders. He works with adult police officers to solve cases and has no qualms about keeping the dead skin of his classmate as a souvenir for solving their murder case. He holds the profession of a detective in high esteem. He always remains calm during cases and is confident that he can solve them, claiming that he has never felt pressured.
Tsukumojuku can often be cryptic, speaking about Beyond and in a metaphorical sense about himself that George and others do not understand. George notes that Tsukumojuku frequently lectured him to think before impulsively throwing himself into danger.
In his own novel, his voice as a baby is way too beautiful that even singing and laughing causes his parents, doctor, and several nurses to faint from hearing it.
Tsukumojuku has a very good sense of smell. Despite the stench of Antonio Torres's rotting skin, Tsukumojuku can smell Lisa Lisa's shampoo from it, as she was hiding inside the skin in order to lure Antonio's killer.
Tsukumojuku has a Beyond, which guided him when he was the protagonist of his own novel. After becoming a secondary character in JORGE JOESTAR, his ability passed on to George Joestar, Jorge Joestar, and Dio Brando.
Tsukumojuku was born in a town called Nishi Akatsuki in Fukui Prefecture, Japan. As a child, Tsukumojuku showed interest in reading detective novels, believing in playing the role destined to "always arrive at the truth." His father's occupation in archaeology allowed him to travel to different places around the world such as Africa. At the age of five, Tsukumojuku moved to Canary Islands with his mother. By the time he was six, he had already solved cases in mainland Spain and Egypt, and became a self-proclaimed detective in the field.
Tsukumojuku formally meets George and Lisa Lisa while solving the case of Antonio Torres' apparent death. Declaring himself a detective, he claims to have solved the case after showing Antonio's corpse to the class, much to everyone's horror. He explains that the corpse was not truly Antonio, but a discarded sheet of skin which was a work of art by Maria Torres, Antonio's mother, after carefully peeling off pieces of her son's skin since he was a baby. While George wonders if the real Antonio is still alive, Tsukumojuku opens the door, revealing Lisa Lisa in Antonio's skin. Later on, using Lisa Lisa's plan to trap Antonio's true murderer, the trio is attacked by Alejandro Torres, Antonio's father, who informs them that he had already eaten his son's body in the morning and is now targeting Lisa Lisa next. George steps in the battle and knocks himself out in an attempt to overcome his cowardliness. Tsukumojuku visits George during his recovery, and they soon become best friends.
In the next four years, Tsukumojuku and George go on many adventures, solving cases on the island such as the mystery of the fifteen locked room murders. They solve the locked room murder case with the help of Penelope de la Rosa, discovering that Javier Cortes is the culprit. With this friendship, Tsukumojuku tells George about his belief in "Beyond", an omniscient force that controls destiny which George dismisses as mere delusions. Shortly after, he informs George that he has to depart for his home in Fukui, Japan. George hopes that they will meet in Japan one day, but Tsukumojuku places his hope in Beyond instead. Three days after his departure, Tsukumojuku's boat vanishes off the coast of Florida, with the boat having been found at the bottom of the sea three months later.
Not long after the boat's vanishing, Tsukumojuku finds himself in the company of Jorge Joestar in a different Fukui Prefecture, Japan. Jorge meets him after saving him from nearly drowning in Serika Kato's house, which was strangely filled with seawater. Tsukumojuku introduces himself and says that he had been on a boat before ending up in the Kato's household. He then finds out that his own home address is the same as the Kato's, even though he is not related to them, and is shocked that the boy who saved him was also a detective named Jorge Joestar.
In the hospital, Jorge and Tsukumojuku discuss about the map of their worlds. The geography of Jorge's world is completely different from his, where there is only one continent, Panlandia, and the Canary Islands, the Atlantic, and the Kingdom of England do not exist at all. He also tells Jorge about the other Jorge in his world, who helped him solve the case that Jorge happens to be solving as well. The two assume that Tsukumojuku somehow ended up in the year 2012 in a parallel universe through the Bermuda Triangle. After he helps Jorge crack the case of the fifteen locked room murders, Jorge receives a call from the police who warns him that, in the dreams of the victims in his case, they had been told that if Jorge comes to Morioh, he will be killed.
Shortly after Jorge leaves Tsukumojuku at the hospital, Tsukumojuku heads to Morioh during the night. He enters Arrow Cross House, which belongs to Rohan Kishibe. He stumbles into Cube House, a time traveling tesseract located in a room below the study. Tsukumojuku uses it to time travel multiple times and then once he returns, he is promptly killed by a zombie Antonio Torres, who had secretly swapped places with the skin sample that Tsukumojuku was carrying with him.
The next morning, Rohan finds Tsukumojuku's corpse in his study. His naked body, wrapped only in a diamond-shaped red cloth, is discovered mounted on a stuffed polar bear with a slit in his throat and a broad axe slung over his shoulder, resembling the folk tale character Kintarō. As it turns out, Tsukumojuku had been the victim of a series of murders involving two other detectives, whose bodies were made to resemble Japanese folk tales after their deaths. Jorge and Rohan attempt to investigate his murder.
A Brief Reunion
From the effect of Cube House, Tsukumojuku's spirit returns to George Joestar to assist him once again. After Darlington Motorize promptly tells George to leave the house, Faraday, the Motorize family's butler, tells him that a friend of his was just waiting for him in the house. He was a young boy who looked like he was in middle school and spoke Spanish at first, fitting Tsukumojuku's description.
Faraday brings George to another room and he finds Tsukumojuku there, although floating in the air. He tells George that he is not really in the room but in Japan in the year 2012. He brings up the Beyond and says that it was George's Beyond that willed him to appear in front of him. Tsukumojuku grabs George's hand before saying that they will meet again one day, and then he is suddenly teleported to a dark area, holding Lisa Lisa's hand instead. Later on, while battling a creature with Lisa Lisa, George realizes that Tsukumojuku brought him there because Lisa Lisa needed him, as she was afraid.
Back in Morioh, Jorge Joestar tries to find out who the mysterious figure is that he saw wandering inside Cube House. He checks every room only to find that they are all empty. Reimi then screams after checking the camera that was installed to surveil the figure. The group sees Tsukumojuku Kato captured on the screen merely a few minutes ago, despite being dead. In the footage, after he hears the sound of the collision of Nero Nero Island and Morioh, he walks up to the camera and laughs, saying that he guesses his life entirely belongs to the Jorges, before vanishing again.
Tsukumojuku later appears to assist Jorge for his travel to Mars in spirit. After meeting Giorno Giovanna and his team, Jorge is hired to be his detective to locate Diavolo, Passione's boss on Nero Nero Island. Narancia Ghirga is assigned to be Jorge's assistant when Tsukumojuku suddenly appears standing next to him. He grabs Jorge's shoulder but Narancia gets in the way, and in that instant, the three of them are teleported on H.G. Wells, the spacecraft orbiting Mars. Tsukumojuku comments on bringing an "anomaly", although he figures that it may have a meaning, and vanishes once more.
In George's universe, a few years after his encounter with Tsukumojuku's spirit, Lisa Lisa and George reunite and profess their love for each other. Lisa Lisa reveals to him that she became worried after the Tibetan named Tonpetty secretly prophesied that George would die around the time they get married. She starts crying but George promises that he won't die and leave her alone. After speaking with Tonpetty, George begins to accept his fate, but Tonpetty tells him that there is a "god" looking out for George and if George believes in this power, then he won't die. On the other hand, if he can't put his faith in it, he will be brutally murdered by something terrifying. George realizes that this god must be what Tsukumojuku believed to be the Beyond, and that he must believe in it. Later, after the 920,000 Antonios invade England and attack George, George reminisces about Tsukumojuku. As he starts to believe in his Beyond, he manages to summon Tsukumojuku's spirit one last time, while Antonio is shocked to see him alive and still fifteen years old. Tsukumojuku teleports George to 2012 in Morioh, leaving Antonio, the zombies, and England behind. Tsukumojuku tells George that it was an honor meeting him and says his final goodbyes.
- “I call myself a detective, and I perform the role well. I have my failures, and make mistakes, but I remain calm. Why? Because I am certain that I am a detective, and will solve the case. That is why I never hesitated to call myself a detective. You understand? In my mind, the word 'detective' is an honor. One others use to describe you, not something you ordinarily use to describe yourself. Artists don't normally call themselves masters, or geniuses. They don't describe their own work as masterpieces. Describing yourself as a detective is normally just as comical.”
- “I've never felt pressured. I always find the truth.”
- “I don't believe the world was made for me; I know it was. I'm not talking about the small boost to confidence than comes when things are going well for you. I mean I was chosen by the god of this world. And I know that I was. That's why I can call myself a detective without a trace of shame, and have that cause no problems at all.”
- “Say that I'm Sherlock Holmes. Then I'm certain that I have something outside of this world serving as my Arthur Conan Doyle. I am as certain of this as I am certain that I am a detective. And I have a name for this thing guiding me from somewhere not of this world. I call it: Beyond.”
- “Gods do not care about the individual. They don't play favorites. Even if they do give someone a role to play, they would not manipulate matters to the point of disrupting the natural order. What I have has the power of a god, but one that works only for me.”
- “But here I am speaking in the present tense, when really, this is all in the past. Like I said at the start, I no longer have the certainty required to call myself a detective. My Beyond has abandoned me. I'm still me, but my role in this world is no longer guaranteed by Beyond. Beyond has chosen a new protagonist for this world... for this story. You, George Joestar. This is the last thing I'm certain of.”
- “That'll never happen. With Beyond at your side, your adventure will be without compare. Let me give you one piece of advice: You should believe. Remember that. Believe in Beyond, and you will overcome your fate.”
- “Lord knows, as a detective myself, I'm sick of explaining to people that because I am there, everything has a meaning. Just like the arrival of a detective in a mystery novel. In a sense, Beyond is a mystery novelist. Beyond is writing a mystery novel in which you are the detective. And you should be aware of that fact.”
- “I exist now in eternity, in the final frontier. But I expect I won't be there forever. There must be some meaning for me to have come here. Like I always said. Everything has meaning. You wanted me here, or something needed me for your benefit.”
- “Ha ha ha, I guess my life belongs entirely to Jorge Joestar. To both Jorges: Don't worry about it. In the words of Iason Sobra Quatro: 'Life is an explosion.' My life is exploding as we speak. Ha ha! See you again.”
- “At last you decided to believe in Beyond, Jorge. It took you getting this bloody? You sure are a troublesome protagonist.”
- An alternate universe version of him is the protagonist of Tsukumojuku, a previous novel written by the same author, Ōtarō Maijō. Another version of him is also a character in the JDC Series by Ryūsui Seiryōin, which the Tsukumojuku novel is based on.
- Tsukumojuku means 9, 10, 9, 10, 9 (九十九十九). Flipping the kanji for 9 (九) creates the astrological symbol for Jupiter, ♃. Jupiter is the Roman name for the Greek god Zeus, known as king of the gods. The kanji for ten, 十, is a cross, so Tsukumojuku's name has three gods (九) linked together by two crosses (十). If God is considered to be the Trinity, then God can be split into three. After his death, Tsukumojuku is with George in England, with Jorge in the Arrow Cross House, and with Dio Brando in his coffin, all at once.
- Tsukumojuku possibly resembles Jesus, based on the appearance of Dio's Beyond. In the Sixth Story of his own novel, Tsukumojuku encounters several of his own disembodied heads. They all have long hair and are wrapped around three times with rope to look like crowns.