Badnik

Badniks are mass produced robots created by Dr. Eggman to serve as common enemies throughout the game. They are powered by little animals which Sonic can free by destroying the badniks. Badniks come in a wide variety of designs and models, suited for the various environments throughout the games.

E-Series
E-Series robots are an advanced line badniks introduced in Sonic Adventure. E-102 Gamma and E-123 Omega are both playable characters part of the E-Series line.

G.U.N.
G.U.N., or Guardian Units of Nations, is a worldwide military organization that were introduced in Sonic Adventure 2.