|Real name||Benjamin Kirby "Ben" Tennyson|
|Nano Item||Sumo Slammer Card|
|Voiced by||Yuri Lowenthal|
|“||It's hero time!||”|
Ben Tennyson is the grandson of Maxwell Tennyson, friend to his former enemy Kevin and cousin of Gwen Tennyson. Ben was an average ten-year old kid, until he accidentally found the Omnitrix, an exceedingly strong and powerful alien wristwatch-like device that allowed him to turn into ten different alien species. With the powers of the Omnitrix, he began to fight criminals around the world, and attracted alien threats too. Ben eventually took off the watch, after not being able to all summer long, and resumed the life of a normal kid.
Five years later, after his grandfather had gone missing investigating an alien plot to destroy Earth, Ben made the hard decision to put back on the Omnitrix and finish the investigation his grandfather had started. As of episode 46 of Ben 10: Alien Force, Ben has destroyed the Omnitrix, which was on the wrist of Vilgax at the time. Ben then took from Albedo the Ultimatrix, an alien device that allows him to transform into any of over 63 accessible sets of aliens found across the galaxy and with a total set of at least 1,000,910 aliens and with the incredible ability to evolve the aliens into their Ultimate Forms, granting them new and stronger powers and abilities.
After the first Terrafusers landed on earth, fifteen-year old Ben (as Four-arms), teamed up with Dexter to defend Foster's Home, Mac, and Bloo from the Fusion Spawn invaders.
With the help of Professor Utonium, Dexter developed plans for an Electronic Pulse Cannon to disable the Terrafusers. Unfortunately, two key parts to the E.P.C. were in the custody of both his nemesis, Mandark, and the villain, Mojo Jojo. Mojo's lair was raided by a team consisting of Ben, Numbuh Two, Bubbles, and Blossom. Ben was able to bypass Mojo's security system as the Galvanic Mechomorph he called "Upgrade." Despite a Fusion Spawn attack, that part was recovered without a hitch, since Mojo and his minions had abandoned the lair in their attempt to conquer Planet Fusion. Ben (once again as Swampfire) then helped defend Dexter's ship against the Fusion Spawns in order to buy him and Professor Utonium time to fire the cannon. However, due to a miscalculation by Mandark, the cannon fire was too late to stop the Terafusers from summoning Planet Fusion and its army to Earth.
Ben is having more problems than usual with the Omnitrix, which started acting strange as soon as Planet Fusion appeared. This can be seen in the FusionFall trailer, with Ben running away from a Horror King, while trying to turn into one of his aliens, only to be rescued by a passing hero on a zipline. With a basic understanding of how the Omnitrix works, Dexter helps to keep the Omnitrix stable when it starts to go haywire. When Ben is able to use the Omnitrix, Fusion Gwen always appears to steal a piece from his alien hero forms, such as a talon from Jetray, fur from Spidermonkey, a stone from Chromastone, or a swamp root from Swampfire, presumably in order to create Fusion versions of them. Later it was shown that Fuse had also stolen a scale from Humongousaur, and had earlier acquired a barbell from Four Arms. It is revealed by Paradox that Vilgax is the one who figured out how to disrupt Ben's Omnitrix (with the unintentional assistance of the hero) by somehow blocking the Omnitrix's signal from Primus. Together with Paradox, Azmuth, and the Ben 10 crew, the hero will try to get Ben's Omnitrix (Ultimatrix) back to normal.
- Ben is unique as he is the only Guide with more than one Nano of himself. Though his standard human form is not an available nano, six of his aliens are.