Oh, Japan. Well, I guess we inadvertantly offend people when we name stuff, too!
Mr. Satan, or Hercule, or Hercule Satan, or Mark (really. Hercule and Satan are his stage names. "Mark" is his real name, because in Japanese it becomes an anagram of "Akuma") is a martial artist. In real-world terms, he is AWESOME, and pretty much Chuck Norris. But Dragon Ball Z isn't the real world, now, is it? As far as DBZ is concerned, he is by far the weakest major character.
See, while everybody was saving the universe from Saiyans, Frieza, and Androids, Mr. Satan was busy winning the World Martial Arts Tournament and becoming a famous hero. So, when Cell declared the Cell Games, Satan challenged him to a fight... and got tossed into a mountain pretty quickly. But he ended up taking all the credit after Gohan defeated Cell, and became famous enough to get a city named after him! He really came to prominence in the Buu saga, where despite everything, Mr. Satan pretty much saved the day twice.
Mr. Satan is an egomaniac, a glory hog, and kind of a fraud and a coward - he KNOWS who the heroes are, but still keeps taking all the credit. But his heart is ultimately in the rigbht place - he DID redeem Fat Majin Buu, after all, and even went toe-to-toe with Kid Buu for a little bit, despite knowing how outclassed he was. Mr. Satan's gift of gab and reputation also help the heroes a lot - when Goku needed more energy for hi super-duper Buu-destroying Spirit Bomb, Mr. Satan was the one who convinced the remaining people of earth to give their energy. And Gohan eventually marries his daughter.
One of Mr. Satan's big public trademarks is how he can break an inordinate number of bricks with a single karate chop. BUT... he always fails. No matter how many or how few bricks or boards he's got, there is always one left unbroken. Poor, poor guy.
And yeah, he's not devilish at all. But to avoid offending people, in some dubs (French, the censored US one), he is renamed "Hercule." But most (almost all dubs, the uncut US one) keep him as Mr. Satan. The uncut US dub actually just calles him Hercule Satan and uses them interchangably, which works... though I have to wonder why they didn't make him "Mr. Hercules." That actually would have fit him pretty well.