E-101 Beta, more commonly known as E-101 ONE, is the second robot in the E-100 Series of Sonic Adventure, after E-100 Alpha. He is the main antagonist of E-102 Gamma's story.
E-101 Beta Mark I appears after Gamma's functions are tested, and he proves that he is working properly. Dr. Eggman decides to have the two robots duel, because of lack of space on the Egg Carrier. Beta is bested by his "younger brother", and Eggman decides to reconstruct Beta. Gamma later discovers Beta while he's being reconstructed and is horrified, and this ultimately is a factor in Gamma's rebellion against Eggman.
Beta himself doesn't return until the very end of Gamma's story. Fully reconstructed, E-101 Beta Mark II flies from the insides of the crashed Egg Carrier and begins to fight E-102. After a heated battle, Beta blasts Gamma, with both of them exploding, releasing their Flicky power sources, which are family members.
- Of the E-100 Series robots seen onscreen (Alpha, Beta, Gamma, Delta, Epsilon, Zeta, and Omega), Beta is one of the only two who doesn't speak, the other being Alpha. Delta, Epsilon, and Zeta each have one line when presenting the wrong frogs to Eggman.