The character at the center of the anime Dangerous Jii-san Ja is simply known as Grandpa, or Jii-san. He is an elderly man whose frail and harmless appearance completely contradicts his unpredictable and overwhelmingly chaotic nature. Rather than a typical grandfather figure, he is a self-proclaimed expert on survival who believes it is his mission to teach the people around him, particularly his grandson, how to navigate the myriad dangers of the world. His methods, however, are far from conventional.

Grandpa’s personality is defined by a bizarre blend of cheerful innocence and relentless mischievousness. He maintains a consistently upbeat and carefree demeanor, often seeming blissfully unaware of the absolute mayhem his actions cause. He is described as a nonsensical, utterly disgusting, and relentless prankster who employs an absurd and surreal brand of humor. His teaching style does not involve lectures or warnings; instead, it is a hands-on, catastrophic demonstration of what not to do, often achieved by inadvertently falling off cliffs, escaping the hidden dangers of sleep, or engaging in petty, over-the-top rivalries. His antics, which can include excessive phlegm and potent flatulence, are designed to frustrate those around him to hilarious extremes.

The primary motivation for Grandpa is to fulfill his self-appointed role as a protector and teacher of survival skills, though his lessons are almost always for his own amusement and at the expense of others. He does not appear to have a grand, overarching goal but is content to turn the daily routines of a quiet town into absurd and dangerous adventures. Whether he is grocery shopping, gardening, or simply taking a leisurely stroll, these mundane activities escalate into chaotic events due to his seemingly uncontrollable and superhuman abilities.

Within the story, Grandpa is the central figure, and each episode revolves around his unpredictable antics and the reactions of his long-suffering companions. His most significant and consistent relationship is with his grandson, Yosuke. As the series' straight man, Yosuke is the primary victim of his grandfather's chaos, enduring physical pain, embarrassment, and utter bewilderment. Their dynamic forms the core of the series' comedy, with Grandpa leading the charge and Yosuke reluctantly acting as his audience and collateral damage. Other key relationships include his rivalry with the hot-tempered school principal, who has a severe inferiority complex and often clashes with Grandpa, only to be defeated by cunning traps. He also shares his home with Gebe, a mysterious cat-like creature whose powers are even greater and stranger than Grandpa's own. The local townsfolk, including a bewildered police officer and an exasperated neighbor named Tanaka, complete the cast of characters who constantly react to Grandpa's bizarre behavior.

The series is largely episodic, with no significant character development for Grandpa. His personality and abilities remain consistent throughout, acting as a constant, unchanging force of nature. He exists not to grow or change, but to disrupt the status quo and provide a consistent source of absurdist humor.

Grandpa’s abilities are as extraordinary as they are ill-defined. He possesses superhuman strength, agility, and resilience, capable of feats like outrunning a speeding car or accidentally demolishing a building. He is also a master of traps and pranks, using his cunning to outwit rivals like the principal. Despite his destructive potential, Grandpa remains blissfully unaware of the true extent of his impact, which is perhaps his greatest and most consistent trait.