Happosai is the elderly founder and grandmaster of the Anything Goes School of Martial Arts, and he serves as one of the primary antagonistic forces within his fictional world. He is an ancient martial arts prodigy, having lived for well over a century; his exact age is a subject of ambiguity, with some narratives suggesting he is several hundred years old. Despite his advanced age, he possesses a small, diminutive, and hunched physical form, which belies his immense power and agility. He is typically bald with tufts of hair on the sides of his head and a thin mustache, often dressed in a traditional martial arts gi or other simple outfits.

The personality of Happosai is a study in extreme contradictions. On one hand, he is an unparalleled genius of combat, a master strategist, and a living repository of martial arts knowledge. On the other hand, he is almost entirely driven by base hedonism, specifically an all-consuming and frankly childish perversion. His days are primarily spent engaged in the pursuit of women's undergarments, which he steals with manic glee, stashing a massive collection in a large sack. He is an inveterate groper and a nuisance to nearly every female character he encounters. This behavior is not merely a pastime but appears to be a physical need; supposedly, going without touching a woman will cause him to become weak and ill. Beyond his lechery, he is selfish, petty, sadistic, and deeply manipulative. He delights in tormenting those around him, especially his former students, and holds grudges with a fervent passion. Despite these despicable traits, he occasionally displays a rare, albeit surprising, benevolent side. He has a soft spot for young children and, in certain situations, has shown a degree of chivalry, claiming a code against fighting women directly. However, these moments are fleeting exceptions to his generally chaotic and self-serving nature.

Happosai's primary motivation is the uninterrupted pursuit of his own pleasure. This encompasses perfecting his martial arts, but more often manifests as the need to steal lingerie, peep, and cause general mayhem. His role in the story is that of a chaotic wildcard. He is introduced after escaping a seal placed upon him decades prior by his students, Genma Saotome and Soun Tendo, who had grown tired of his abuse. He returns to the Tendo Dojo, effortlessly defeats the two men, and essentially invites himself to live there, becoming a permanent, unwanted fixture. He acts as a formidable rival to the protagonist, Ranma Saotome, often proving to be an opponent the young martial artist cannot defeat through sheer force, forcing Ranma to rely on clever tricks and schemes. He is a source of endless conflict, creating situations that range from humiliating pranks to life-threatening perils for the other characters. At the same time, as the grandmaster, he technically holds authority over the dojo's inhabitants, a fact he exploits mercilessly.

Key relationships define much of his interaction with the world. His former students, Genma and Soun, live in abject terror of him, their hatred tempered by the memory of his overwhelming power and their own guilt over imprisoning him. Ranma Saotome despises him more than almost anyone, viewing him as a personal archenemy and a disgrace to martial arts, though they are occasionally forced into uneasy alliances. Perhaps his most significant and complex relationship is with Cologne, the ancient matriarch of a Chinese Amazon village. In their youth, Happosai was infatuated with her, but his relentless lechery and subsequent theft of her village's valuables turned her into a powerful rival. They share a deep history as martial arts masters on a similar level, and their interactions are a blend of grudging respect, mutual animosity, and a shared past of youthful indiscretion. He also has a curious connection to the character Pantyhose Taro, whom Happosai accidentally cursed as an infant by bathing him in a cursed spring, and later named.

While Happosai shows very little personal development or change throughout the narrative, remaining a static embodiment of his defining traits, his existence forces development in others. He is a catalyst for many stories, pushing Ranma to grow stronger and more cunning as the young fighter constantly seeks new ways to overcome or outwit the old master.

In terms of abilities, Happosai is unequivocally one of the most powerful martial artists in the series. His physical capabilities, including speed, strength, and agility, are far beyond what his frail appearance suggests, allowing him to easily defeat nearly any opponent. He possesses an encyclopedic knowledge of lost techniques, pressure points, and magical artifacts. Among his notable techniques is the Happo Fire Burst, a powerful ki-infused firework that causes massive explosions. He can also project his battle aura to paralyze foes, create elemental effects, or even form a giant projection of himself. The Fist of the Hermit Crab allows him to hide in thrown buckets for a surprise attack, and with the Extra-Dimensional Warp, he can become invisible, though he cannot attack in this state. He also possesses dangerous knowledge, such as the Ultimate Weakness Moxibustion, a technique that can permanently rob a fighter of their strength. Despite this unmatched power, his childishness and his perverse obsessions are also his greatest weaknesses; a cleverly baited trap involving the promise of lingerie is often the surest way to defeat him.