During a walk Ben finds the Omnitrix, which latches to his arm. Unable to remove it, he uses its powers to become a super hero. However, he finds the job trickier than he imagined, especially with the evil alien warlord Vilgax pursuing him relentlessly to retrieve the Omnitrix. No matter what bizarre form Ben takes, inside he is still the same mischievous 10-year-old boy. He is something of a glory hog, sometimes taking credit that he does not deserve. He also tends to be a sore loser, especially when it is Gwen, with whom he shares a love-hate relationship who outdoes him. Ben is not above using the Omnitrix to pull pranks, even during urgent situations. In spite of all this, Ben is good-hearted and will stop at nothing to protect his family or anyone in danger. Even when not in alien form, Ben's resourcefulness has saved himself and others from many a dire situation, especially when the Omnitrix cannot activate or does not provide him with the form he originally wants. His most frequent catchphrase, usually said when something goes wrong, is "Aw, man!" His others include "It's hero time!" and "Going hero!"

Ben's relationship with Gwen in the original series is somewhat complex. The two of them bicker almost constantly, can hardly agree with each other on anything (the primary exception being Max's bizarre cooking), and their fights have, on occasion, even led to physical confrontations.

Twenty years into the future, Ben (this version voiced by Fred Tatasciore) has become a full-time superhero. He no longer bothers to conceal his identity, as he is recognized everywhere as Ben 10,000 or Ben 10-K. He also has 10,000 alien forms, hence the name, and has once again unlocked the Omnitrix's master control function, allowing him to switch forms at will. Having been fighting crime for such a long time, Ben 10,000 has grown into a more strict and serious character, though his interaction with his past self in the episode "Ben 10,000" causes him to lighten up somewhat. Ben's alien forms have aged with him, which is evident in their appearances and abilities. For example, Ben 10,000's XLR8 is noticeably taller with grey outlines in place of the ordinary blue, as well as being considerably faster. Ben eventually has a son named Ken, whom he gives an Omnitrix of his own in the episode "Ken 10".

In the sequel, Ben 10: Alien Force, Ben (voiced by Yuri Lowenthal) is a much more mature and resourceful character, having learned the value of teamwork through his role as a goalkeeper for his soccer team; at the same time growing out of his childish antics. However, there is a bit of the old Ben Tennyson in him. Nevertheless, his relationship with Gwen is also improved; they no longer fight, and their quips about one another are more friendly. He now sports a black shirt, a green jacket with a white vertical line and the number 10, while his soccer uniform resembles his original clothing. He found a way to remove the Omnitrix roughly five years before the series began, allowing him to live a normal life. When Grandpa Max goes missing, he teams up with Gwen and a reformed Kevin to find him. The Omnitrix has been recalibrated, giving access to a new set of alien forms; in addition, it reconfigures into a more watch-like shape. It also carries a new weakness: if Ben's aliens are hurt when the Omnitrix times out, so too is Ben hurt as a result. Contrary to the Omnitrix's previous ill-timed deactivations, the new recalibrated Omnitrix appears to deactivate more or less to Ben's will, and can be used multiple times in succession, though its power will eventually deplete and will require time to recharge. Although he, like Gwen, is a descendent from the Anodite race, Ben does not bear their powers.

Gwen Tennyson

Gwen in the original seriesGwendolyn "Gwen" Tennyson (voiced by Meagan Smith) is Ben's ten year-old cousin (the two share the same birthday). Haley Ramm portrays Gwen in the live-action film, Ben 10: Race Against Time.

Gwen uses her laptop to research any strange people and/or creatures they might come across. Gwen shares a love-hate relationship with Ben, insulting him with dry wit and sarcasm most of the time. Gwen is a capable gymnast and has had some training in martial arts.She has also shown an almost innate skill in using various complicated devices or objects, such as in the "What If?" episode "Gwen 10", when Gwen gained the Omnitrix instead of Ben and proved to be far more adept in its use than he initially was.

Overall, her most notable skill is her innate, if latent, ability at magic. Having within her what Charmcaster calls a "magical aura", Gwen is capable of using various magical artifacts and spells. When using the magical, luck-giving Charm of Bezel, she was able to activate it instinctively, whereas Hex did or could not. Later on, when she takes and uses his staff, she was able to make it fire an energy blast ? according to Charmcaster, only someone of Hex's skill should have that ability. Gwen starts tapping into her magical powers when she procures a book of spells from Charmcaster in the episode "A Change of Face".

In the possible future of "Ben 10,000",a (voiced by Tara Strong) combination of physical and magical training leads her to become both a black belt and a master magician using the Charms of Bezel (Gwen has traveled to the Dimension of Bezel where a new set of charms was created for her), which makes her a hero in her own right.

On two occasions, Gwen has used magical items known as the Charms of Bezel to become a superheroine by the name of Lucky Girl. At first, her ability drew from the "luck" Charm of Bezel, which innately gave Gwen perfect luck. While wearing the charm, any action Gwen took would result in an Rube Goldberg-style chain reaction benefiting her original intent but usually injuring Ben in the process. She later brought back her superhero identity after obtaining the Keystone of Bezel, which increased her natural abilities to superhuman levels rather than affecting her luck.Both times, Gwen was forced to retire when the Charms of Bezel were destroyed.

In the sequel, Ben 10: Alien Force, Gwen (voiced by Ashley Johnson) is now a black belt. In this version, Gwen wears a plain, blue shirt with a white collar, a black skirt with black tights, her hair is much longer and darker, and her lips are more visible. Her magical abilities have also grown considerably. She is able to create energy constructs for nearly any purpose, usually creating platforms to levitate herself (and others), beams to grab enemies, and circular blasts to knock enemies back. She also has a wide range of other loosely defined magical abilities, such as dowsing and clairvoyance. Just as Kevin likes Gwen, so too does she like Kevin; however, Gwen is notably more forward about this affection (to the point of bluntly asking why he won't ask her out), while Kevin is less willing to admit it. She gives him a kiss in Episode 8. Her powers are also revealed to be alien in origin. Her grandmother, Verdona, is an energy being known as an Anodite. Verdona tells Gwen that what she originally thought of as "magic" was actually the manipulation of life energy dubbed mana. Verdona offers to take Gwen to her home planet and train her in the mystical ways of the Anodites, but Gwen prefers to remain on Earth with Ben and Kevin.

Max Tennyson

Max Tennyson in the original series.Maxwell "Max" Tennyson (voiced by Paul Eiding), or Grandpa Max as his grandchildren call him, is the 60-year old, rather overweight and a bit slow at times, paternal grandfather of the cousins Ben and Gwen Tennyson. With a keen sense for adventure and a strange taste in food (the only topic about which Ben and Gwen never argue), Max always ends up taking the kids on interesting trips. As soon as school lets out, Ben looks forward to going on a traditional summer road trip with Max. Much to Ben's annoyance, Ben discovers that his cousin Gwen (whom he dislikes) is joining them as well. Max travels in a motorhome nicknamed "The Rust Bucket", which he has modified with advanced technology. The Rust Bucket is a replica of a GMC Classic Motorhome. In the live action Ben 10 film, Max is played by Lee Majors.

Max was once in the Army, had joined the astronaut program, and was a "plumber" (among other things, it seems) and has been through his share of scrapes; he takes his grandson gaining superpowers in stride and supports Ben in any way he can. Ben and Gwen were ignorant of this at first, believing him to have been a literal plumber. They began questioning this after several incidents in which he displayed knowledge and skills that a plumber would not normally have; everything from military-style hand signals to a working knowledge of nuclear reactors. Max and Vilgax are also old enemies, with Vilgax even going so far as to call Max "a thorn in his side."

In reality, Max was indeed a plumber, but not the kind that Ben and Gwen thought he was. He was part of a government organization called "The Plumbers" which dealt with extraterrestrial, paranormal, and other situations that many others could not handle. Their base was beneath Mount Rushmore. The group was disbanded shortly after Max defeated Vilgax for the first time, suggesting that Vilgax had been largely responsible for the various incidents they dealt with. Max was to have been among the candidates eligible for the Apollo 11 mission, and would have gone were it not for other concerns.

By the age of 80, Max has further modified his RV, grown a beard, and lost his arm in the Limax Wars of 2017. In its place, he uses a neuroprosthetic replica of his arm, which can also extend and transform into a selection of futuristic weapons. Further in the future (now 92), around when his great grandson Ken is 10, Max has lost his beard and much of his weight and height, which was explained as being due to his age and a steady diet of futuristic health food.

Max is an old friend of an alien named Xylene. She had intended to send the Omnitrix to Max, but Ben found it by accident. Max and Xylene seem to have been romantically involved and work well as a team. Xylene tries to convince Max to form a partnership and travel across the galaxy with her, but Max eventually turns her down twice: the first time due to his duties as a Plumber, and the second time due to his need to look after the kids.

In Ben 10: Alien Force, Max looks mostly the same as before, save for couple of minor wrinkles and whiter hair. He disappears shortly before the start of the series, leaving only a holodisc with a message for Ben. In the message, he states that he is investigating renewed alien activity on Earth and that Ben need not worry. A follow-up message erroneously claims that he has the Omnitrix, followed by an image of a DNAlien flickering at the end. In "Kevin's Big Score", Kevin tracks down a second holodisc containing a message from Grandpa Max. This one, while still cryptic, does not contain any hints of deception as the first does. In it, Max advises Ben to form a team to fight the alien threat, recommending that he track down other child Plumbers. He appears in person in "Max Out", where he infiltrates a Highbreed factory to rescue Ken Tennyson, and is captured by him, partially mutated into a DNAlien. He is reunited with Ben and Gwen when they come to free him. In order to stop the factory from mass-producing DNAliens, Max uses a Null Void Projector (minus the focusing lens) to destroy everything within a half-mile radius.