Ben Tennyson is a teenager from a parallel universe who wields the Ultimatrix, formerly the Omnitrix. He arrived in Rex's world through a space-time rift.
Ben is 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 filled with alien DNA which had fallen from space. He, his friend, Kevin, and cousin, Gwen, are part of a group called the Plumbers.
When Ben first arrived in Rex's universe, they both got into a brawl as Ben mistook Rex for an alien while Rex mistook him for an EVO. Ben was eventually captured by the Providence Defect Group, but he 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 this point, Ben met Caesar, who had come to save them both from Alpha, a dangerous creation made by Caesar six years ago during the Nanite Project.
After Alpha turned hostile, Ben used Cannonbolt to escape with Rex from Alpha. The two went back to The Plant where they played basketball and explained their origins. However, Alpha attacked the EVO containment to absorb the nanites from the present EVOs, including Rex. Ben helped save Rex from Alpha, causing him to flee.
Ben wields the power of the Ultimatrix — a powerful alien 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. It also allows him to "evolve" his aliens into their ultimate forms.
Please note that these are only the aliens seen in "Heroes United".
Humungousaur is a Vaxasaurian from the planet Terradino. 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.
Diamondhead is a Petrosapien from the planet Petropia. 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 blades, as well as firing sharp crystals as projectiles at his opponents. Additionally, he is capable of using his crystals defensively, such as creating crystal walls to protect himself and his allies.
Lodestar is a Biosovortian. 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.
Rath is an Appoplexian from the planet Appoplexia. He is a humanoid alien that is similar to a bipedal tiger. He has superhuman 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. He believes all problems can be solved by hitting it (or sometimes, hitting it a lot). He also apparently lacks the strong sense of shame most Appoplexians have, as he technically runs around nude.
Big Chill is a Necrofriggian from the planet Kylmyys. He 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.
Cannonbolt is an Arburian Pelarota from the once beautiful planet, Arburia, which was destroyed by The Great One. He 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 a Kineceleran from the planet Kinet, and 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 Tetramand from the planet Khoros. He 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 ponytail. He wears gold gauntlets on his arms and wears long black pants with a gold belt. He wears the Ultimatrix symbol on his chest.
Shocksquatch is a Gimlinopithecus from the planet Pattersonea. 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, unlocked during the battle with Alpha in the Bug Jar.
Upgrade is a Galvanic Mechamorph from Galvan Prime's moon, Galvan B. He is a humanoid alien that 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, including nanites. His species was created by the Galvan by accident.
Murk Upchuck is a Murk Gourmand from the former planet of Peptos XI, currently Peptos XII. He is a reptilian-like being who can digest, and then expel, energy and solid matter, even if said energy or matter is much larger and heavier than himself. Ben Tennyson used this power to carry the Alpha Nanite's compressed sphere out of Rex's Universe and back into the Null Void, as he headed back to his universe
In a flashback, Ben is seen with many of his other aliens in the background. The ones that are unmentioned are Wildmutt, Ripjaws, Stinkfly, Heatblast, Way Big, Goop, Chromastone, Brainstorm, Jetray, Spidermonkey, Echo Echo, and AmpFibian.