Jubei Kakei is the heir to the Kakei clan, a family that has served the Fuuchouin clan for five hundred years as doctors and bodyguards. This long-standing alliance is central to his identity, defining his role and his profound sense of duty. He is a childhood friend of Kazuki Fuuchouin, and their bond is so deep that when Kazuki was forced to flee to the Infinity Fortress, Jubei followed without hesitation, accompanied by his older sister, Sakura.

Jubei is characterized by an impossibly serious and stoic demeanor, which often places him at odds with the more modern and lighthearted attitudes of other characters. His speech is formal, reminiscent of a samurai from feudal Japan, earning him the nickname Samurai Boy from Ban Mido. True to this archetype, he has virtually no sense of humor and frequently fails to understand jokes. After being told he lacks humor, he makes earnest but often awkward attempts to be funny, which only further cements his reputation as a comically serious person. His loyalty is absolute; his primary motivation is to protect Kazuki, a mission he considers his sworn duty and is willing to die for.

In the story, Jubei is a prominent figure within the Infinity Fortress, initially serving under Kazuki in the legendary gang Fuuga. He later becomes one of the elite New Four Kings, serving Makubex. His most significant personal conflict arises when he feels betrayed and abandoned by Kazuki, who left the Infinity Fortress. This leads to a brutal confrontation during the IL retrieval arc, where Jubei tragically loses his eyesight. However, the battle ultimately reconciles them, and their close relationship endures, with Jubei remaining fiercely protective of Kazuki.

Jubei has a deep but often competitive relationship with his fellow bodyguard and former Fuuga comrade, Toshiki Uryuu. While they bicker constantly, especially over Kazuki, there is an underlying bond of loyalty and mutual respect between them. He is also extremely protective of his older sister, Sakura Kakei, coming to her defense whenever she is threatened.

Jubei experiences significant development, primarily through his physical struggles and his strained relationship with Kazuki. After losing his sight, he develops the ability to sense the flow of chi, allowing him to perceive movement and identify people without needing to see them. He regains his eyesight in the Get Back the Lost Time arc after a battle with Ginji Amano, during which he saves Jubei. Later, when confronting the Ura-Fuuchouin clan, he endures having his heart ripped out by Yohan, only to be saved by Kazuki, solidifying that their bond transcends all previous conflicts.

Jubei is known as Tobari no Juubei or Juubei of the Flying Needles, as his primary weapons are long needles. He is a master of the Kakei needle techniques, which are rooted in acupuncture and the understanding of chi flow. By inserting needles into specific pressure points, he can perform feats such as turning his arms as hard as steel for protection. He also has the ability to heal wounds using his needles. His most dangerous and forbidden technique involves the Kokushin or Black Needles, which are made of magnetic lodestone. By generating a powerful magnetic field with a Taichi Stone, he gains precise control over these needles, but at a great cost: the process inflicts severe damage on his own body. His overuse of this ability was the direct cause of his blindness. Later, he takes the drastic step of implanting the Taichi Stone into his own body to further increase his power, though this causes the stone to emit radiation, making it dangerous for him to be near others.