Mark (or Mr. Satan or Hercule, depending on the versions) is a main character in Dragon Ball Z and Dragon Ball GT.
The Tenkaichi Budokai champion, Mr. Satan is widely known all over the world as the strongest man alive and a celebrity hero. He is the father of Videl, the father-in-law of Gohan, and the maternal grandfather of Pan. He first appears during the Cell Saga, to fight Cell who started attacking the planet. However, Satan is too much weak (especially compared to the Z Warriors) and gets thrown out of the ring without even hurting his "opponent," who merely paid attention to him.
During the following fights, Mr. Satan pretends to be ill, and that all the others are cheating. He, however, accepts to take Android 16's head to Gohan, which proves useful, since it convinced the boy to fight Cell with all his power. After the fights are over, Satan takes credit for killing Cell, and is labeled a hero.
Years later, Satan is called again to fight Majin Buu, and again is too weak to defeat him. Instead, he manages to become Boo's friend, and convinces him to stop killing. Though Boo later mutates into an even more dangerous monster, neither him nor Satan forget about their previous friendship. Thanks to this link and to his newfound courage (mostly when he starts believing the whole story is a simple dream) Satan becomes a suprisingly important ally to the Z Warriors, helping them to defeat Pure Majin Boo once and for all.
Satan again takes credit for the victory, but still remains friends with the fighters and the good Boo, who comes to live with him.
Satan is the strongest "normal" human of the series, and has an incredible luck, since he not only survives all the dangers he goes through, but also gains fame for most of the heroes' acts. Though he is extremely arrogant, cunning, a good liar and often clueless about what is going on, he still is very good-hearted and heroic when he has to be.
He is voiced by the late Daisuke Gōri and the late Unshō Ishizuka in the Japanese version of the anime, and by Chris Rager in the English version of the anime.
Mr. Satan is a tall man of a rather muscular and hirsute physique. His black curly Afro, mustache, and well trimmed sideburns are among his most recognizable traits, as well as his cleft chin. Mr. Satan typically is seen in his martial arts uniform or in fitted suits, but has been known to wear more luxurious clothing when resting or making public appearances. His gi is often accented with either a red or white cape depending on which gi he dons, and his notably hairy chest and arms are usually displayed openly.
In the end of Dragon Ball Z and throughout Dragon Ball GT, His black curly afro is now in half as the center of his head is naturally bald and a few wrinkles due to his age.
During the Dragon Ball Z time period, Mr. Satan is the reigning World Martial Arts Champion, a title won fairly by beating other normal earthlings in a time when Goku and other powerful fighters were battling on Namek and later against the Androids. During the time of the Cell Games, Mr. Satan was already rich and famous, but also an egomaniac. He boasted endlessly on television about his strength, and that he was the "best fighter in the world." In the eyes of characters who know otherwise, he is considered to be a showboating blowhard. When the Z Fighters arrives on the scene starting with the Cell Games, he starts a pattern of claiming them to be his disciples, which, while preserving his status and self-serving ego, has produced good results in the past. Mr. Satan is quite an unrepentant lech whose head is easily turned by a pretty face (not unlike Master Roshi and Elder Kai), and he selfishly uses his fame as "the Hero who defeated Cell" to seduce and sleep with multiple women after his wife Miguel left, causing a rift between him and his daughter Videl (though as Videl implied, some of them may have initiated the seduction). It is also implied during the Cell Games, that Mr. Satan may have had an affair with his manager Miss Piiza at some point based on some of Miss Piiza's comments. His lecherous tendencies are also demonstrated by the fact that he owns a copy of Bob & Margaret, a controversial adult book (a copy of which was also owned by Master Roshi) which Majin Buu later found and read inspiring Majin Buu's fission that lead to the creation of Miss Buu and eventually gave raise to the Majin race after Majin Buu and Miss Buu used the book to develop the Love-Love Beam technique. In some ways this makes his personality similar to Master Roshi's as both are lecherous yet heroic martial artists, the only difference is that Mr. Satan is apparently far more successful with women than Roshi due to his fame and wealth. Despite his lecherous tendencies, he apparently does not encourage said behavior in others as he is shown trying to convince Majin Buu to return the book in Xenoverse 2 after he discovered his friend had found it in the Majin Buu's time rift anomaly, stating that it wasn't the type of book someone like the pure hearted Buu should be reading indicating that he was try to preserve his friend's innocence, though eventually gave up when Majin Buu stubbornly refused (ironically despite this Majin Buu never creates Miss Buu in the anomaly's timeline as Buu was inspired by Mr. Satan's relationship with Videl to create his children via fission instead).
He is very protective of his daughter Videl however, and would not allow her to hang out at any boy's house unless they are tougher than him (which he might consider). After the Majin Buu conflict he accepted Gohan's relationship with Videl, presumably due to Gohan's power and likely due to Mr. Satan accepting him as being worthy of his daughter, due to Gohan being the true winner of the Cell Games, which Mr. Satan had taken credit for (as well as being aware of Gohan's good nature). Similarly, he also was very protective of his granddaughter, Pan, as evidenced by the 28th World Tournament: When Pan managed to beat Wild Tiger, his very first action upon her winning was to scramble onto the ring towards the locker rooms in a panic and ask if she was alright, and also was constantly worried about her during the match.
In spite of showing many signs of greed and cowardice, Mr. Satan is still a fundamentally honorable, decent, sensitive and kind-hearted man. He genuinely cares about protecting the innocent and fighting for justice to the best of his abilities, and is prone to violent and righteous anger when faced with the abuse of helpless innocents by villains. He shows moments of great courage when his family and friends are threatened by evil. Mr. Satan's true colors are foreshadowed when he risks his life to get Android 16's head near Gohan, which leads to Gohan's transformation to Super Saiyan 2. In the Majin Buu Saga, Mr. Satan is revealed to be a fundamentally good man as opposed to the arrogant showboat persona he exhibits, and his bravery and kindness towards the good Majin Buu play a major role in Kid Buu's defeat at the end of the Kid Buu Saga. Prominently, he has saved the lives of Bee, Vegeta, and even Majin Buu (recognizing that Majin Buu was used in his innocence by Babidi, he tearfully begged Vegeta to spare his life as he laid helpless after being beaten by Kid Buu, and was ready to risk death to protect him; Mr. Satan's love for his friend moved Goku enough to join in his defense, allowing the formerly evil monster to start a new life as a hero and benefactor to society). Mr. Satan's skill as an orator came into good use by the series' end. Without the adoration and support of his legions of followers, the Z Fighters would never have been able to raise the Super Spirit Bomb and defeat Kid Buu. By the end of Dragon Ball Z, Goku and his friends come to see the good in Mr. Satan and decide that he is a noble hero.
Following his acceptance by Goku and his friends, Mr. Satan showed his graciousness through his monetary generosity, not only giving Goku enough money to pay for Goten's schooling, but also in far more dramatic expressions of his wealth and good will as well.
Despite his egotistical personality, Mr. Satan is extremely generous towards his daughter and extended family, such as building a massive mansion-style house for Videl and Gohan as they were starting out as a married couple and as parents. Mr. Satan believes that his family deserves the best and only the best, and often acts as financial benefactor for his loved ones. He is much the family man-type, and upon discovering his daughter's pregnancy he was overjoyed and proudly proclaimed that he was going to be a grandpa. He has a very close and affectionate relationship with Pan, often cuddling and hugging her whenever he gets the chance. He also is proud of her abilities, and is frequently dumbstruck by her talents, which eventually causes him to see her as his potential successor to carry on his legacy in Dragon Ball GT.
He and Goku may differ substantially, but they share a very special respect for one another: Mr. Satan admires and respects Goku's otherworldly might despite his fear of it, and Goku in turn respects Mr. Satan's humanity and kindness, which was enough to tame Majin Buu, one of the most terrifying monsters in the universe.
Mr. Satan's greatest strength might just be his extraordinary humanity and kindness. His surprisingly gentle heart is often seen in his interactions with Majin Buu, the dog Bee, and his extended family. This gentleness, however, belies his profound hatred for senseless murder and violence. This was most prominent in his clear disgust of Van Zant, who nearly killed Bee in an effort to provoke Buu. Mr. Satan's righteous fury was on full display as he dispatched the mad gunman, his hatred readily apparent.
While self-serving at times, Mr. Satan's personality truly boils down to a well meaning goofball whose overwhelming ego has gradually receded with age and experience, showing the hero he has always wanted to be inside to the world. He believes in his friends and family, and will even look past his own safety in the face of comically overwhelming odds when pushed past his limits. In many ways, he represents the best that the common human has to offer in the face of the outlandish beings of power he often is in the company of.
Although very weak when compared to Goku and the others, Mr. Satan is a skilled martial artist whose strength exceeds that of regular people.His strength is much greater than that of "normal" humans like the evil gunmen Van Zant and Smitty, and it is higher than that of a martial artist like Spopovich (before Babidi increases his power). Mr. Satan also seemingly has superhuman levels of strength and speed, demonstrated when he moves so fast he vanishes from sight making it appears like he is teleporting (to Van Zant - an ordinary human), pulls four buses single-handedly and punches a hole in one (in the anime), tearing multiple phone books in half, and crushes fourteen tiles simultaneously with a chop. In his match against Spopovich in the 24th World Tournament (only in the anime), he displays the ability to leap very high into the air and land with no apparent damage or difficulty, and effortlessly sends Spopovich, who is much larger and more muscular than him, flying with a single kick. Mr. Satan also survived a fight with Mercenary Tao (albeit with extreme injury) while training at Satan Castle (surviving a battle with Tao which resulted in the death of his martial teacher), something not even the likes of General Blue or Bora were capable of. However, according to Gohan, his skills are below that of his daughter Videl, who has a basic understanding of ki and flight, and takes training more seriously than he does.
He is able to survive being knocked into a mountain by Perfect Cell (although Piccolo notes that Cell did not want to expend any effort into attacking the likes of Satan), and even a light punch to the face from Kid Buu, though these instances are likely intended for comic relief. In Dragon Ball GT, he also survives a kick from Omega Shenron, though again for comedic effect and, like with Cell, it's possible that Omega Shenron didn't exert any real effort into the attack.
All in all, Mr. Satan's fighting capabilities are similar to other normal Earthling martial artists seen in Dragon Ball like Nam and King Chappa who while not as strong as Goku and the Z Fighters (who, with all fairness, either have access to training a 'typical' human would generally never have or aren't human) are still stronger than normal untrained humans and possess superhuman capabilities such as superhuman strength and speed (allowing them to use techniques like Rapid Movement and the Afterimage Technique).