Benjamin Kirby "Ben" Tennyson was a normal ten-year-old kid until he found the Omnitrix, a powerful watch-like device that allowed him to turn into ten different aliens. Though initially immature and clumsy, Ben grew to be a true hero. Ben has also been deputized by the Plumbers and gained more aliens as time progressed.
Ben's physical appearance has changed throughout the series from a ten-year-old to a sixteen-year-old. However, he retains some recurring characteristics. He has shaggy brown hair, green eyes, pale skin, and he's quite skinny for his age, though he shows a lot of muscle definition due to being athletic.
In the original series and Omniverse flashbacks, Ben's usual clothes consisted of a black-striped, white shirt with short sleeves, military green cargo pants with side pockets, and black and white sneakers with black stripes. Ben wore the prototype Omnitrix on his left wrist.
In Multiverse Force and Omniverse, Ben wears a black shirt that has a green stripe in the middle with a white 10 and green stripes on the sides of his shirt. He wears brown cargo pants with pockets at the knees and green and white shoes. He later gains a white hooded jacket with green stripes on both arms and a green number 10 on the right side for some time. He wears the Omnitrix on his left wrist.
In Omniverse, while riding the Tenn-Speed, he wears a white helmet with the number 10 on it, a green visor, a short-sleeved black T-shirt, long white pants and a belt with the number 10 on it.
initially cocky, childish, and arrogant, Ben's immaturity and attention-seeking behavior often led him to joke around, regardless of the situation. His allies often criticized his attitude, unaware of the fact that he used humor to mask his fears. Despite some childish attitude, he was heroic, caring and generally good-natured, always willing to save others at any time.
Over the years, Ben has displayed good leadership skills, as well as the ability to adapt his attitude to a situation, becoming serious when it's called for. He became more mature, responsible and sensible. He's kind-hearted, willing to sacrifice even his own life for others. His idealistic views and unwillingness to compromise his values – aspects condemned by Azmuth, yet commended by others – sometimes drive him to act against reason, such as helping his enemies if they need it (most remarkably Lord Vilgax). Professor Paradox has praised Ben's good nature, going so far as to say he had the gift to make the right choices at the right moments.
Though often stated to be silly or unintelligent because of his immaturity, Ben is cunning and resourceful when needed, quickly adapting when the Omnitrix doesn't provide him with the alien he wanted.
Later on, Ben starts thinking of himself as more of a superhero than a Plumber, often remarking that he's not a cop, but a superhero. Although he once again became cocky, he retained his selfless and heroic nature.