E-121 Chi is a character mentioned in the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics. It was the twenty-first unit in the E-100 Series line, created by Dr. Ivo "Eggman" Robotnik.
E-121 Chi was created sometime between 3236 and 3237, possibly during or after Dr. Eggman's campaign on harnessing the power of Chaos. According to the Eggnet, it has been destroyed, presumably by E-102 Gamma, but due to a memory error the robot has no recollection of this happening.
- In the video game Sonic Battle, the robot designated as E-121 was Phi rather than Chi. According to Ian Flynn, this misnumbering is the real-world reason for Gamma's memory error.