Jousuke Udono is a member of the Kouga Manjidani clan, known for his jovial personality and remarkably corpulent physique. He is one of the ten ninja selected by his clan's leader to compete in a deadly, centuries-old conflict against their rivals, the Iga Tsubagakure clan. A loyal friend and a straightforward warrior, Jousuke often serves as a foil to the more serious and reserved nature of his comrades.

Physically, Jousuke is an extremely fat man with short brown hair and a beard, typically wearing a large brown vest and light-colored shorts. A distinct circular marking sits between his eyebrows. His most defining trait, however, is his unusual body, which possesses the consistency and properties of rubber. This unique constitution is the foundation of his fighting style, granting him several formidable abilities. His body can deflect physical attacks and even catch a blade with his flesh, allowing him to absorb strikes that would fell a normal opponent. He can also squeeze his pliable form through impossibly narrow openings, such as the bars of a prison cell. Furthermore, Jousuke can inhale vast amounts of air to inflate himself like a balloon, enabling him to bounce like a ball, roll over enemies as a living boulder, or float into the air. Despite his heavy appearance, he possesses surprising speed and agility, allowing him to dodge attacks and move nimbly even in mid-air.

Jousuke’s personality is defined by his good humor and lecherous tendencies, often landing him in trouble. He is unabashedly honest and speaks his mind freely, frequently making crude jokes or unwelcome advances towards women, particularly the Iga kunoichi, Akeginu. During a tense dinner with the Iga, he drank heavily and boldly proposed marriage to her, much to the anger of everyone present. His boorish demeanor, however, belies a more perceptive nature. He is often the first to notice when circumstances are amiss, and during the Kouga's stay in the Iga village, he was the first to realize their hosts were planning something suspicious. His primary motivation throughout the story is his unwavering loyalty to his clan leader and close friend, Gennosuke Kouga. He remains by Gennosuke's side during their journey to the Iga village and acts as a protective and sometimes comic companion.

His relationship with Gennosuke is the most significant of his alliances. He follows Gennosuke's commands with respect and cares deeply for his safety, checking on him after escaping confinement. In contrast, his infatuation with Akeginu is one-sided and ultimately dangerous. He constantly flirts with her, attempts to grab her, and even agrees to a duel for the chance to spend a night with her, not fully grasping that she intends to kill him. Despite her murderous intent, he covers for her when they are discovered, attributing her hostility to mere annoyance. During his journey, he also taunts the Iga elder, Rousai Azuki, using playful insults to goad the serious old man into a rage.

Jousuke’s role in the story is that of a faithful retainer whose arc culminates in tragedy. After the rival clans' fragile peace treaty is secretly annulled by the shogunate, the Iga are given free rein to kill their Kouga guests. While bathing after his inconclusive duel with Akeginu, Jousuke discovers the semi-liquid form of the Iga ninja, Jingorou, who had been stalking Gennosuke. Jousuke confronts him and demands to know why he would attack under a banner of peace. In a moment of shock upon learning the truce is broken, his guard drops temporarily. Jingorou exploits this vulnerability, forcing his liquid body down Jousuke's throat as he attempts to take a deep breath. Suffocated from within, Jousuke’s body, now a lifeless floating shell, becomes a casualty of the renewed hostilities, demonstrating that his trusting and straightforward nature could be exploited by more underhanded techniques.