Ben is a typical 10-year-old boy - he plays video games, rides his bike, and sometimes gets into some mischief. During a camping trip with his grandfather Max and his cousin Gwen, he finds the Omnitrix, an alien device that gives him the ability to turn into various aliens. Unable to remove it, he uses its powers to become a superhero. He finds the job trickier than he imagined, especially with the evil alien warlord Vilgax pursuing him relentlessly to retrieve the Omnitrix. However, no matter what bizarre form Ben takes, inside he's still the same mischievous yet good-hearted 10-year-old boy.

Ben's 10-year-old cousin, Gwen (the two share the same birthday), is the voice of reason that often unsuccessfully tries to keep him from trouble whenever Ben is ready to dive headfirst into a bad situation. Ben usually wants to ignore her and thinks that (from his point of view) she has no idea when to stop nagging him. Gwen is always trying to plan ahead and calculate the consequences of their (mostly Ben's) actions, because she is convinced that Ben never thinks before he acts. She uses her laptop to research any strange people and/or creatures they might come across, and continually tries to help Ben figure out how the Omnitrix works (much to her own dismay). Although, most of the time, she holds Ben in contempt and insults him with dry wit and sarcasm, resulting in the two of them bickering a lot, she has some respect for him deep down but she dislikes showing it.

Gwen has shown an almost innate skill in using various complicated devices or objects. When using the luck-giving charm of Bezel, she was able to activate it instinctively, whereas Hex didn't (or couldn't). Later on, when she gets ahold of his staff, she is able to make it fire an energy blast, which (according to Hex as told to Charmcaster ), only a master magician should be able to do. In the out-of-continuity, "What If?"-styled episode "Gwen 10," she gained the Omnitrix instead of Ben, and proved to be far more adept in its use than he was or still is.

Max Tennyson, or Grandpa Max as his grandchildren call him, is Ben's and Gwen's grandfather. With a keen sense for adventure and a strange taste in food, Max always ends up taking the kids (and their taste buds) 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 dismay, Ben discovers that his cousin Gwen (whom he dislikes) is joining them as well. Max, who was in the army (among other things, it seems) and has been through his share of scrapes, 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 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 my side." In actuality, he 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 extra-terrestrial, paranormal, and other situations that others could not. 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.

 

A vicious, intergalactic conqueror from a currently unknown alien species and unspecified planet, Vilgax will stop at nothing to obtain the Omnitrix and use it to build an army capable of conquering every planet in the galaxy. After being severely injured after a final attack from the ship that was transporting the Omnitrix, he was confined to a regeneration tank, ordering his robotic minions and cosmic bounty hunters to track down the Omnitrix, not realizing that Ben outsmarted and outfought his minions. However, while his body was being repaired, he was also being cybernetically enhanced. When he was finally able to leave the regeneration tank, he had been enhanced to the point that none of the Omnitrix's aliens could so much as scratch him. Vilgax's ship was destroyed with him still aboard, but because he survived every attack from the Omnitrix's aliens and a sizeable fall puts his demise into question. His ability to survive the destruction of his previous ship from a nuclear missile, at which point he had not been enhanced, casts further doubt on his demise.