X Yamakawa is a Japanese high school student who cultivates a self-proclaimed outcast and delinquent persona, yet expresses confusion over his resulting lack of friends despite deliberately isolating himself. His family includes an ultra-nationalist mother and a military-oriented brother, Seitarou Yamakawa, both of whom regard him as a profound source of shame due to his rebellious behavior and failure to meet their expectations. This dynamic provokes extreme reactions, including attempts on his life when he brings perceived dishonor, such as appearing in public wearing an absurd costume.

His interactions with the protagonist, Techno, frequently make him a target for harassment and experimental technologies. Early on, Techno forcibly attaches an electrocution collar to him, linked to a cat's movements, subjecting him to unpredictable shocks if he strays too far. Later, he receives an unwanted dinosaur suit via mail that fuses to his body upon wearing. This predicament prompts his family to declare him a disgrace and actively attempt to eliminate him, forcing him into hiding as a fugitive mistakenly perceived as a monster by authorities and the public. During a confrontation with a giant slug mutation, he briefly gains social acceptance when classmates encourage him to battle the creature, though this validation proves fleeting.

A significant shift occurs when he forms an unlikely bond with Goro, a giant spider created by Techno. Following shared confinement alongside other characters in a cabinet during an android confrontation, he emerges with a genuine friendship with the spider, signaling a move away from self-imposed isolation. Despite remaining socially awkward and prone to embarrassment—such as fleeing after being seen in his underwear—he gradually transitions from a purely antagonistic role toward Techno into a more integrated, albeit eccentric, class member. His narrative arc concludes symbolizing broader societal reform themes within the story.

Throughout the series, his attempts to shed his delinquent image and connect with others consistently generate situational humor. His family's exaggerated responses to his antics, including his mother's melodramatic fear that he might "destroy Tokyo Tower," further cement his role as comic relief. Despite his quest for acceptance, he retains a harmless nature, with his rebelliousness manifesting more as a performative identity than genuine malice.