Born Takeshi Gouda, the character known as Jaian derives his nickname from the English word giant, a direct reference to his large and imposing stature. He is a local neighborhood boy recognized for his robust and often overweight build, which contributes to his physically intimidating presence among his peers. His family life is central to his identity; his mother runs a small local grocery store and is one of the few people who can control him, as he is genuinely terrified of her wrath. In contrast, he shares a loving and protective relationship with his younger sister, for whom he has a notable soft spot. His family is depicted as being of more modest means compared to some of his friends, and he is often shown helping out at the family shop.

Jaian’s personality is defined by a powerful combination of aggression, overconfidence, and a deeply ingrained sense of entitlement. He is the self-proclaimed leader of the local children, ruling through physical force and bullying, with Nobita being his most frequent target. He is notorious for forcibly borrowing toys, manga, and other possessions from his friends under a self-serving interpretation of sharing, rarely returning anything unless it is already broken. Despite his brutish nature, he possesses a significant blind spot regarding his own talents. He believes himself to be a gifted singer and a talented chef, but in reality, his singing voice is ear-shatteringly awful and his homemade dishes, like his signature Jaian Stew, are notoriously inedible and grotesque. He enthusiastically subjects the entire neighborhood to his prolonged concerts and culinary experiments, and any criticism or attempt to avoid these events is met with threats of violence.

His primary motivation is the maintenance of his dominance and the gratification of his own desires, which often includes forcing others to participate in his hobbies or attend his performances. He is driven by a fear of appearing weak or unpopular, which can sometimes lead to moments of introspection. On the rare occasions he realizes his friends are avoiding him, he makes clumsy, and ultimately unsuccessful, attempts to become kinder, only to revert to his old ways when his efforts are mocked. In a broader sense, his motivations also include a genuine, if roughly expressed, desire to succeed in sports and a deep-seated care for his loved ones, especially his sister.

In the story’s dynamics, Jaian primarily serves as an antagonist and a source of conflict for Nobita, whose problems with the bully often prompt him to seek help from Doraemon. He is the physical powerhouse of the group and the captain of the local baseball team, named the Giants after himself, where he forces his less-skilled teammates to play and frequently blames them for losses. However, his role shifts significantly in the theatrical films, where he almost always becomes a courageous and dependable ally. In these adventures, his immense strength and fighting spirit are turned against common enemies, and he is often one of the first to support Nobita and take charge in a crisis.

His key relationships are complex. His interactions with Nobita are the core of his role as a bully, yet this dynamic is layered with a fierce, unspoken loyalty; he will relentlessly torment Nobita on a regular day but will go to great lengths to protect him when he is in genuine danger. He keeps company with Suneo, who acts as a willing and sycophantic sidekick, though Jaian does not hesitate to turn his aggression on him as well. He has a more respectful and distant friendship with Shizuka, who is often horrified by his behavior and his cooking. The single most defining positive relationship is with his younger sister, Jaiko. Despite his thuggish exterior, he is fiercely devoted to her, often going to great lengths to support her dreams, particularly her hobby of drawing manga.

While Jaian’s character shows relatively little permanent development in the episodic series, there are recurring themes of occasional self-reflection. Stories sometimes explore his loneliness when his friends avoid his birthday party, prompting a temporary realization that his selfish behavior is the cause, leading him to briefly try to reform. These attempts are almost always short-lived, as his short temper gets the better of him. His most notable development is the duality of his nature: while he is a cruel antagonist in everyday life, he consistently demonstrates growth into a brave and reliable friend during the long-form movie adventures, suggesting a hidden depth and a true, if flawed, sense of honor.

Jaian possesses notable abilities that extend beyond his physical strength. He is by a wide margin the strongest and most athletically gifted child in his peer group, excelling in sports, especially baseball where he serves as the team's captain and star player. His physical prowess makes him a formidable force in any confrontation. However, his abilities also include a bizarre talent for causing unintended chaos through his abysmal singing voice, which is powerful enough to be weaponized, and his horrifying cooking, which can function as a form of biological warfare on anyone who tastes it.