Ben Tennyson is a teenager who wields the Omnitrix, now Ultrimatrix from an alternate universe that arrived in Rex's world through a space-time rift.
Ben is known to be a living legend in his universe due to his use of the powerful Omnitrix/Ultimatrix. When he was ten years of age he found the Omnitrix, a watch that was filled with alien DNA, which fell from space. He and his friend and cousin, Kevin and Gwen, are part of a group called the Plumbers.
When Ben first commenced into Rex's universe, they both got into a battle that ended up badly. He was eventually captured, but escaped trying to find a way back home. Once he finally realized that there was no way back he gave up and decided to assist Rex in his time of need. At one occasion, Ben met Caesar, who had come to save them both from Alpha, a dangerous creation made by Caesar.
Ben Tennyson is a sixteen year-old male with green eyes and brown hair. He normally wears a green jacket with white stripes and a "10" on it. He also wears a black t-shirt underneath along with blue jeans and sneakers. Most noticeably, he wears a green gauntlet-like watch on his wrist, addressed as the Ultimatrix. In all of his alien forms, he has green eyes and the Ultimatrix badge symbol on his chest.
In a flashback, a ten-year-old Ben has a white t-shirt with a black stripe in the center.
Ben wields the power of the Ultimatrix — a green, level 20 [alien] technology-wise, and powerful watch that resembles a gauntlet. The Ultimatrix grants him the ability to change into various extraterrestrial lifeforms, thus granting him a wide variety of powers and abilities within each and every transformation. It also allows him to "evolve" his aliens into their ultimate forms.
Please note that these are only the aliens seen in Heroes United. For more of Ben's aliens, please visit the Ben 10 Wiki.
He is a large, brownish alien creature that possesses incredible strength and dinosaur-like features, hence his name. He can also grow up to a certain extent in size, with an additional increase in strength.
He is an alien that has crystalline features and is able to control and grow crystals at will. He is able to reshape his limbs, such as turning his arm into a sharp blade, as well as fire sharp crystals as projectiles at his opponents. Additionally, he is capable of using his crystals defensively, such as creating a crystal wall to protect himself and his allies.
He is an alien with a floating metal head in between two sharp shoulders that provide themselves as a magnetic field. He has crab-like claws for hands and feet and his body is mainly black with yellow feet, hands, shoulders and chest. With this alien, Ben possesses the power to control magnetic forces, moving metals and other magnetic objects of all different types.
He is a humanoid alien that is similar to a bipedal tiger. He has enhanced strength and agility. Like a tiger, he is orange with black stripes. He can also retract two short blades from behind his knuckles that allow his punches to be more effective. Rath, like all other Appoplexians, is also known for his arrogance and aggressiveness, both verbally and mentally.
Big Chill is a humanoid, moth-like alien whose wings and antenna can fold up into a hooded robe, giving him an appearance similar to a phantom. He has a black body with blue and white spots, resembling ice chunks, on his limbs. He is capable of exhaling a freezing vapor that can instantly encase his targets in a thick layer of ice. Big Chill can also render himself intangible. He can combine these abilities to freeze objects he passes through. He can release an ice vapor mist when exhaling, freezing whatever it comes in contact with.
Cannonbolt is a round alien that has the ability to curl itself into a ball. He can accelerate himself in this state, allowing him to burst off very quickly at his opponents. He has a yellow "shell" which enables him to defend himself. His body is mostly colored white with black stripes.
XLR8 is one of Ben's fastest aliens. He has a black body with a blue striped tail. He wears a spiked helmet with a blue visor. Without the visor, he has green eyes, black lips, and a blue face. He has wheels on his feet, and leaves a track of black and blue-stripped lines while he runs. He has a white stripe down his chest and wears the Ultimatrix symbol on it as well.
Four Arms is a four-armed, built alien with four eyes. He is one of Ben's muscular aliens, like Humungousaur. He is red-skinned and has a pony tail. He wears gold gauntlets on his arms and wears long black pants with a gold belt. He wears the Ultimatrix symbol on his chest.
He is an alien with a yeti-like appearance. He is able to manipulate and emit strong yellow electric currents from his mouth. He is also one of Ben's newer aliens.
Upgrade is a humanoid alien. He has black skin with green mechanical prints throughout his body. With this alien, Ben possesses the ability to take, manipulate, and enhance anything and mostly everything technological. An animation error means that his Ultimatrix symbol is missing from his chest.
Upchuck is a reptilian-like being who can digest, and then expel, energy and solid matter. Ben Tennyson used this power to carry the Alpha Nanite compressed sphere out of Rex's universe and back into the Null Void.
In a flashback, Ben is seen with many of his other aliens in the background, the ones that were unmentioned are Wildmutt, Ripjaws, Stinkfly, Heatblast, Way Big, Goop, Chromastone, Brainstorm, Jetray, Spidermonkey, Echo Echo, and AmpFibian.