E-100 Alpha, better known as ZERO (sometimes with the added title "The Chase Master" after the name of his song on the soundtrack) is a large red-eyed floating green robot that resembles a trash can with large arms that tries to recover Chaos Emeralds and appeared in Sonic Adventure, as the antagonist of Amy's story.
Although he is not called an E-Series robot in the games or Sonic X, he was listed as "E-100 Alpha" on the Sonic Adventure website accessed via Dreamcast. Unlike the other E-100 series robots it does not have "E-100" on its body, but instead it has "E" on both sides of his body. Also, the headlight on ZERO is on his shoulder, in contrast to the other E-100s in which it is on their mid-section. ZERO is also not powered by an animal like the other E-100 robots, as nothing is seen escaping from its body after its destruction.
ZERO differed greatly from the other E-100 robots as he lacked a projectile weapon of any sort, mainly relying on his extending fists to attack his foes. It also possessed impressive strength, able to smash through walls in pursuit of Amy. He also had strong defensive strength, as Amy could only inflict damage by electrocuting him. He pursues his target with a single-minded tenacity, even after the Flicky's Chaos Emerald had been removed. This also made it possible to trick him, as Amy could hide beneath barrels or around obstacles to elude him.
In Sonic Adventure, ZERO made his debut by chasing a blue Flicky in the possession of a Chaos Emerald that has taken refuge with Amy Rose. ZERO chased Amy around Station Square, Twinkle Park, the Egg Carrier, Hot Shelter, and finally, the Final Egg base in the Mystic Ruins.
When he went back to the Egg Carrier, he saw Amy and the Flicky with the bird's relatives, who were inside E-101 Beta and E-102 Gamma. ZERO then attacked and knocked out the small Flicky with an extending punch. Amy, thinking that ZERO had killed her companion, became furious and attacked ZERO- finally destroying him by knocking him into an electric fence, scrambling his circuits, before smashing the vulnerable circuitry in his head with her hammer, causing him to explode.
Other Game Appearances
Sonic Advance 2
ZERO also seems to make a brief appearance in Sonic Advance 2, chasing the player around the pseudo-3D special stage. The manual however, describes him as Dr. Eggman's Guard Robot but he looks exactly the same. If he hits the player, they lose ten rings.
Sonic Rivals 2
ZERO appeared in Sonic Rivals 2 as collectible card No. 81. It was unlocked by obtaining the high score in Zone 6 Act 3.