1) Zero is the prototype for the E-100 series. Zero is E-100 Alpha. Then there's E-101 Beta, E-102 Gamma, etc (following the Greek alphabet).
2) Ticked off at the other robots for not finding Froggy, Eggman gives Gamma some actual missions, and leaves the other E-bots to protect the areas that'd most likely be trouble. Zeta was upgraded to protect Hot Shelter.
--True Red & SA Tails
2) Ticked off at the other robots for not finding Froggy, Eggman gives Gamma some actual missions, and leaves the other E-bots to protect the areas that'd most likely be trouble. Zeta was upgraded to protect Hot Shelter.
--True Red & SA Tails