Who is E-123 Omega

E-123 Omega is a Sonic character, first introduced in Sonic Heroes. He's a robot (created by Eggman) in the E-100 series of Eggman robots. He's only seen around Rouge the Bat and Shadow the Hedgehog, as they are an extremely tight group, since they teamed up as Team Dark in Sonic heroes. He's only been in a handful of games, for he's a more minor antihero. Games he's appeared in are Sonic Heroes, Shadow the Hedgehog, Sonic the Hedgehog (2006), Sonic Chronicles: the Dark Brotherhood, and Mario and Sonic at the Olympic Winter Games (as a non-playable boss).
Ever since he was locked in Eggman's abandoned base with Shadow (who was encased in a special freezing chamber kind of pod), he has held a great grudge towards Eggman, and has vowed to destroy not only Dr. Eggman, but his robot army. In Sonic Chronicles, he even went out of his way to defeat Eggman when the party of characters approached him. Tails attempted to assure Omega that Robotnik was on their side, but Omega assumed that Tails had gone off the deep edge, and attacked him, as well. It took the support from Rouge and Shadow to calm him down.
People often get mixed up between Omega and his "older brother", E-102 Gamma, who share a similar color scheme, as well as the distinction from all other robots as having free will. The funny thing is, they don't look very similar, and they both act different: Gamma will fight or destroy only what is in the way, whereas Omega will destroy anything that can be destroyed, basically.