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. By the time he was sixteen, he had become famous in not only his hometown of Bellwood, but on Earth and beyond. Ben has also been deputized by the Plumbers and gained more aliens as time progressed.
He is the main protagonist of Ben 10: Universal Showdown
Ben now wears a green variant of his shirt and pants, and wears his UAF pants as well. Ben also acquired a new Green and Light-Green Omnitrix, The Omnitrix Mark 5. He still wears it on his left wrist.
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 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.
However, when Ben fails to save someone, or if someone gets hurt because of his failures, he becomes angry and much more violent. Perhaps the most notable of these instances is when Kevin willingly mutated himself to succeed where Ben had failed and stop Aggregor, Ben was set on killing Kevin, even fighting Gwen when she refused to help him. He also threatened to hunt down and destroy the Forever Knights once making Driscoll promise to cease the hunting of aliens.
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.
After Eon assembled every villain in the universe, Ben started to act more mature and less immature. He knew what was at stake and didn't let fame go to his head, like the last few times. When Ben recieved Revonnah-Ben, He once again became cocky and immature, but after losing Revonnah-Ben, Ben became mature again.