Ben Tennyson (known to most by the mononym X) is an evil version of Ben Tennyson from an unknown timeline/dimension
X mostly resembles 16 year-old ben from Omniverse but he is closer to Ben 10,000's height and build with a deeper voice. He wears a black tank top, a black jacket, brown cargo pants and black boots. He wears a modified version of the Omnitrix on his left arm. It resembles Ben 10,000's Omnitrix in the original series but it fully covers his fingers, has a red dial on his wrist, knuckle spikes and vein-like scars stretching to his shoulder.
X's personality has very few similarities to Ben's. X is incredibly sadistic and enjoys inflicting pain on others. He is highly intelligent and cunning, for eight years he acted as a hero while also secretly carrying out villainous actions. X is not insane but rather has a different view of reality.
Much of X's history, including his real identity as Ben Tennyson, is a closely guarded secret known only to his inner circle. What is known is that he never became famous.
Powers and Abilities
Even without his modified Omnitrix, X is a master in armed and unarmed combat. His modified Omnitrix doesn't possess a time out function and X will remained transformed until he chooses to time out or loses consciousness.