The Visitor

During a trip in St Louis, Ben and Gwen meet a female alien named Xylene, who locates the Omnitrix and confronts Ben under the impression that he had stolen it. But when Max comes to defend them, he and Xylene recognize each other as old comrades and sweethearts. The two reminisce later that night, and Xylene reveals that she was the commander of the ship bringing the Omnitrix to Earth when Vilgax attacked it, and that she had intended to send it to Max for safekeeping; Ben received the Omnitrix by mistake. But when one of Vilgax’s remaining drones attacks and captures Max, Ben and Xylene reluctantly team up to save him, with Xylene unlocking Upchuck to defeat the drone.