Gian, whose full name is Takeshi Gouda, stands as a central figure distinguished by his imposing physical stature, aggressive demeanor, and tendency to bully others, particularly Nobita Nobi. His defining catchphrase, "What's yours is mine and what's mine is mine," reflects a domineering attitude that fuels frequent acts of appropriation, such as forcibly taking toys or manga from other children under the guise of borrowing. Despite his intimidating exterior, he occasionally displays moments of vulnerability and self-reflection, like questioning his unpopularity after a poorly attended birthday celebration or attempting self-improvement upon realizing his negative impact on peers, though these efforts often prove short-lived due to his volatile temper. Significant personality flaws include extreme overconfidence in his terrible singing voice and poor cooking skills, which he inflicts on others unaware of their awfulness, and a win-at-all-costs mentality as captain of the neighborhood baseball team, the Giants, where he often blames losses on weaker players like Nobita while forcing their participation.

Family dynamics reveal additional layers: Gian harbors profound fear of his mother, who owns a local grocery store and dominates him through physical punishment when he shirks responsibilities or bullies others. He demonstrates intense protectiveness toward his younger sister, Jaiko Gouda, supporting her comic-writing ambitions by coercively using Doraemon's gadgets, though this sometimes frustrates her. He owns a dog named Muku, whom he rarely tends to but demonstrates underlying care for, despite outwardly calling the pet stupid.

Gian's relationships with other characters are complex. He and Suneo Honekawa maintain a close yet unequal friendship, with Suneo often acting as a sidekick or messenger; however, Gian occasionally turns on Suneo, who sometimes retaliates using stolen gadgets. With Doraemon, Gian alternates between terrorizing the robot cat and collaborating with him, particularly concerning Jaiko's welfare. His interactions with Shizuka Minamoto are comparatively respectful, as he halts aggressive behavior when she intervenes, though she dislikes his bullying and avoids his concerts or meals. Nobita remains Gian's primary target for physical and verbal abuse, driven by an unexplained compulsion Gian describes as being unable to "eat or sleep" without bullying him; however, this coexists with moments of genuine loyalty, such as rescuing Nobita from peril in movies or offering medicine when he falls ill, suggesting an unspoken bond beneath the hostility.

In alternative scenarios like the parallel universe, Gian's counterpart is female and embodies opposite traits: cowardly, gentle, and possessing exceptional singing talent. In feature films and specials, Gian often transitions from antagonist to ally during crises, such as aiding in space battles or helping defeat supernatural threats, showcasing his physical strength and occasional selflessness. His development across media remains incremental, with core traits like aggression and loyalty persisting despite sporadic introspective moments.