Juubei Kakei is the heir to the Kakei clan, a family that has served the Fuuchouin clan for centuries as both physicians and bodyguards. This lifelong duty defines his existence, as his primary purpose is to protect Kazuki Fuuchouin, the heir of the clan his family serves. The two were childhood friends, and their bond was forged early in life. When the Fuuchouin main clan was attacked and destroyed, Juubei managed to get Kazuki to safety, and they fled together to the Infinity Fortress, a lawless and sprawling city within the Shinjuku district. There, in the fortress, Juubei served as Kazuki’s second-in-command in a legendary gang called Fuuga, which later became part of a larger group known as the Volts.

When Kazuki made the decision to leave the Infinity Fortress, Juubei did not follow. He remained behind with his older sister, Sakura Kakei, and under the influence of the young computer genius MakubeX. This decision created a painful rift between the two lifelong friends. They eventually met in battle during the mission to retrieve a powerful item known as the IL. In this confrontation, Juubei lost his eyesight. However, the battle also led to their reconciliation, and they have remained close ever since.

Juubei is an impossibly serious person, whose formal speech patterns resemble that of a samurai from feudal Japan. This leads to him being nicknamed "samurai boy" by Ban Mido. He has a very rigid personality and often cannot understand jokes. Even the series' most notorious teller of bad jokes, Haruki Emishi, insists that his own humor is superior to Juubei's attempts. This lack of humor means that Juubei is often the unintended target of jokes, particularly those relating to his intense and devoted relationship with Kazuki, which is a frequent subject of in-universe teasing and fan speculation.

Juubei is known as "Tobari no Juubei," or "Juubei of the Flying Needles," due to his primary method of fighting. He uses long needles, a skill that is part of his family's heritage. He is a master of acupuncture and a technique known as shinjutsu, allowing him to understand the flow of chi in a body. He can use his needles to attack by throwing them with deadly accuracy, or to heal by inserting them into specific pressure points. He can also insert needles into his own body to temporarily increase his strength or harden his arms for defense.

Under extreme circumstances, Juubei resorts to his most secret and forbidden technique, the use of "Black Needles." These needles are made of lodestone, a magnetic material. Using a powerful magnetic stone called a Taichi Stone, he can control these needles with far greater precision than his standard white needles. However, this power comes at a great cost. Generating the magnetic field damages his own body. His excessive use of this technique is what caused his blindness during his battle with Kazuki in the IL retrieval arc.

He eventually recovers his eyesight during the "Get Back the Lost Time" arc. In this story, he battles Ginji Amano, which forces Ginji to revert to his powerful and destructive alter ego, Raitei. After being saved from Raitei by Ban Mido and MakubeX, Juubei later takes a stand with his sister Sakura and his rival-turned-comrade Toshiki Uryuu to protect his friends at the gate to the Beltline. Following this, his loyalty to Kazuki is tested again when he faces the Ura-Fuuchouin clan. He has his heart ripped out by an enemy named Yohan but is ultimately saved by Kazuki. Later in the manga, Juubei travels with Kazuki to Babylon City, an alternate version of Shinjuku. In this world, they are not warriors but ordinary citizens living a peaceful life, with Juubei serving as a lecturer in acupuncture at a local college.