Happosai is the founder and grandmaster of the Anything Goes Martial Arts style, a centuries-old martial arts genius who taught Genma Saotome and Soun Tendo in their youth. He appears as a tiny, bald old man with white hair on the sides and a white mustache, typically wearing a purple coverall. His exact age is not specified but is implied to be well over a hundred years, ranging from over a century to several centuries depending on the version. In the past, his two students eventually managed to imprison him by getting him drunk, chaining him in a barrel, and sealing him inside a cave with explosives and a barrier, but he later escaped and returned to the Tendo dojo to continue his teaching.

Happosai’s personality is almost entirely defined by his extreme and obsessive perversion. He spends most of his time stealing women’s undergarments, groping female characters, flipping skirts, and peeping. His perversion is directly tied to his power: if he cannot regularly touch women or their clothing, he becomes weak and suffers withdrawal symptoms, sometimes collapsing. Conversely, his lust fuels his martial arts abilities, making him a formidable opponent. Despite his small stature, he is deceptively strong and is considered the most powerful pure human martial artist in the series. He is also mischievous, selfish, and gleefully sadistic toward his students, often tormenting Ranma, Genma, and Soun by framing them for his own crimes or leaving them with huge bills.

His motivations are simple: to satisfy his desires for women’s underwear and physical contact, to enjoy himself at others’ expense, and occasionally to pass on his martial arts knowledge—though he does so in a coercive and abusive manner. He has no genuine interest in bettering his students; he sees them as tools for his amusement. He takes up residence at the Tendo dojo under the pretense of training Ranma, but mostly uses it as a base for his panty raids.

In the story, Happosai serves as a recurring antagonist and comedic foil. He is a source of constant trouble for the main cast, particularly for Ranma (especially in his female form) and Akane. He is rarely a serious threat to life, but his actions disrupt the household and neighborhood, forcing Ranma and others to try to stop him or reclaim stolen items. He also acts as a deus ex machina at times, possessing knowledge or techniques that can resolve certain situations—though always with a price or a twist.

Key relationships include his former students Genma and Soun, whom he trained brutally and who fear him. With Ranma, he is both a reluctant teacher and a primary target of his harassment. He is also a former lover of Cologne, an elderly martial arts master from China, though their relationship ended due to his incorrigible perversion. He shows a brief attempt at reform when he falls in love with a young teacher named Midori, but his addiction ultimately overcomes him, and he loses her respect.

Happosai undergoes little genuine character development across the OAVs. He remains consistently lecherous and unchanged, though occasional glimpses of his past as a martial arts master or his loneliness appear briefly. His abilities are remarkable: he is a master of ki manipulation, can create energy blasts, and possesses a variety of techniques such as the Happo Fire Blast (often a firecracker trick) and the Happo Dai Karin. He also uses bombs and demonstrates superhuman speed, strength, and durability. His main weakness is his addiction—if denied contact with women or feminine garments, he rapidly loses strength and can be defeated. He can also be outsmarted or tricked by exploiting his obsessions.

Overall, Happosai is a comedic villain whose blend of absurd perversion and genuine martial arts prowess makes him a memorable and disruptive presence in the Ranma ½ OAVs.