Happosai is the ancient, lecherous, and overwhelmingly powerful founder of the Anything-Goes School of Martial Arts. Physically, he is a diminutive old man, extremely short in stature and nearly bald except for hair on the sides of his head, with a thin mustache. He typically wears a simple dark brown outfit with a black belt and is almost always seen carrying a tobacco pipe. Despite his elderly and frail appearance, he is incredibly agile, fast, and energetic.

His background is deeply rooted in the history of the series' martial arts. As a young man, he traveled to China for training, where he encountered a young woman who would later be known as Cologne. During his youth, he was already known for his troublesome behavior, hitting on women and stealing their valuables, which led to his rejection and being blasted away by Cologne using a powerful technique. He is the original master who trained Genma Saotome and Soun Tendo, though his training methods were cruel and exploitative, treating his disciples more as slaves and accomplices to his petty crimes. Eventually, Genma and Soun grew so tired of his behavior that they tricked him into drinking too much sake, sealed him in a cave with explosives, and left him there for many decades. He eventually escaped after a thunderstorm broke the seal and made his way to the Tendo Dojo, forcing himself back into the lives of his former students.

Personality is the most defining aspect of Happosai, dominated by three core drives: improving his martial arts, being a nuisance, and satisfying his perversion. He has an extreme, almost pathological obsession with women's undergarments, which he steals constantly, carrying his collection in a large sack. He is also an avid peeping tom. This behavior is not merely a hobby but a physical need; if he goes too long without contact with women or their undergarments, he grows weak and ill. Beyond his lechery, he is selfish, childish, mischievous, and deeply sadistic, especially towards his male students and Ranma, whom he delights in tormenting with cruel pranks and outright attacks. He is a compulsive liar, often fabricating stories about his past to paint himself in a better light or claiming stolen treasures were gifts from old lovers. However, he displays occasional flashes of a different nature. He can act like a sweet, harmless grandfather around women he admires, such as Kasumi Tendo. He also shows a genuinely benevolent side when dealing with young children and once helped deliver a baby. He refuses to fight women, allowing himself to be beaten without striking back. Despite his evil deeds, there is a childish and even pathetic quality to him that prevents the other characters from truly hating him.

Happosai's primary motivation is the pursuit of his own pleasure, whether through martial arts mastery, mischief, or the fulfillment of his perverted desires. He is also interested in finding a worthy successor for his martial arts style and sees potential in Ranma Saotome, whom he constantly tests and "trains" often as an excuse for more abuse. He shows no real remorse for his actions and feels justified in his behavior.

His role in the story is that of a chaotic wildcard and a recurring antagonist, though not a villain with grand, world-threatening plans. Upon his return, he takes up residence at the Tendo Dojo, becoming a persistent source of conflict and comedy for the entire household. He is frequently the instigator of plots to steal lingerie, spy on the female characters, or simply cause trouble. While he is often a nuisance, his immense power means he is rarely successfully stopped by force. He serves as a reminder of the absurd and often sordid history of the Anything-Goes school and a living obstacle for Ranma to overcome, not just in combat but in maintaining his dignity.

His key relationships are defined by his long history. With Cologne, he has a complex dynamic of rivalry, fear, and a past romantic connection, as he once fell in love with her but ruined it with his behavior and theft. He remains terrified of her to this day. With Genma and Soun, he is the cruel master they both fear and hate for the torment they endured, yet they also owe their entire martial arts foundation to him. He treats them with utter contempt and as tools for his schemes. With Ranma Saotome, he sees a potential heir and a new plaything, subjecting him to humiliating training and constant harassment, though he also pushes Ranma to grow stronger. He also has connections to other characters: he is the godfather of Pantyhose Taro, having accidentally cursed him and given him his ridiculous name. He gave Hinako Ninomiya her special abilities as a child, which later became a major weakness for him. He also briefly trains Tatewaki Kuno in the anime, using him as an unwitting accomplice in his underwear thefts.

Character development is minimal for Happosai, as he is largely a static figure of chaos. The narrative does not seek to reform him. Any moments of kindness or vulnerability, such as his backstory with Cologne or his care for children, serve to add a layer of complexity to his otherwise irredeemable nature rather than to signal a change. He remains the same lecherous, powerful old master from his introduction to the end of the series.

His notable abilities are extensive, making him one of the most powerful characters in the series. As the founder of Anything-Goes Martial Arts, his skills in speed, strength, and technique are virtually unmatched. He can create powerful explosives like the Happo Fire Burst, a bomb capable of knocking out top-tier fighters. He can use a Vanishing Technique to become invisible and intangible. He can generate whirlwinds with the Hurricane Rice Ball technique. His Pipe Whip technique uses his pipe to redirect an opponent's force against them. He has a highly developed Battle Aura, which he can use to freeze enemies, fire energy blasts, or enlarge his own body. He also knows the Hermit Crab Fist, a technique that exploits his small size. His experience grants him knowledge of ancient techniques, forgotten pressure points, and alchemical recipes. His only real weaknesses are his own perversion, which makes him easily trapped or distracted by anything involving women, and the powers he himself granted to Hinako Ninomiya, which can defeat him completely.