There are few things in this world that can reasonably be called eternal: Death. Taxes. And one shriveled midget pervert old man with more martial arts skill in his pinkie than you have in your entire body. Happousai is less a man than a force of nature, a typhoon of depravity and wickedness leaving nothing but the lamentations of men and the screeches of woman in his wake.
Little is known about the man called Happousai. He is a pervert, and has been so for quite some time. He journeyed the world seeking out ancient mysteries of martial arts and mystic artifacts, then stealing them and molesting every woman in a five kilometer radius. At some point he took on two students, Soun Tendo and Genma Saotome, whom he proceeded to mistreat severely and not teach very much to at all. Finally, the cowardly duo tricked him by getting him drunk, then sealed him in a cave, blew him up and surrounded the cave with mystic symbols against evil. But this was not enough to stop Happousai. Perhaps nothing is.
Happousai is a very predictable person. He is a man of simple vices. He likes women, he likes their underwear, and he likes even more their reactions to his stealing their underwear. He has a sense of entitlement which leads him to making haughty demands from his 'lessers' (ie, everyone) and he tends to react with either sadistic anger or disdain when his demands are not met. Happousai takes a perverse delight in the discomfort of others, often going out of his way to be obstinate and actively malicious for petty little reasons. He feigns an air of innocence even after the most annoying escapade and acts indignant if accused of wrong doing.
Basically, he's an asshole.
Thankfully for Happousai, he is also perhaps the best martial artist on the planet. His size belies a tremendous spirit. With his aura alone, Happousai can shatter buildings and cow the weak-willed. Using a mastery of leverage, he has a strength beyond his size, and is so fast that even trained prodigies of the martial arts cannot see his attacks. While he is physically fragile, and can be taken out of action with but a few blows, he is amazingly resilient. Massive explosions, cave-ins, attacks which can shatter stone... Happousai recovers from all but the most damaging attacks practically instantly (and often, whenever it would be most inconvenient). In short, Happousai is an unholy terror, a force to be avoided or endured rather than defeated.