Ben Tennyson is a teenager 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.