E-123 Omega or, technically, E-123 Ω, is a robot created by Dr. Eggman, and is out to destroy him due to lack of respect and want for revenge. He bears a strong resemblance to another one of Eggman's robots, E-102 Gamma (a possible design inspiration), who is part of the same robotics production line. He has the capital Omega symbol (Ω) painted on the front of his left shoulder, in contrast to all the other E-100s, which had the lowercase Greek letter (ω), possibly representing that he is the most powerful of Eggman's E-Series.
Quest for revenge
Omega was made as the 24th and last of Doctor Eggman's E-100 series robots. He was designed by Eggman to prevent the release of Shadow the Hedgehog, and, in case he failed to control Shadow when he is freed. To accomplish this, Eggman sealed him in an abandoned base with Shadow. Being imprisoned infuriated Omega; he decided to prove his status as the most powerful E-Series robot by abandoning his original programming and taking revenge on his creator and the other machines he built.
When Rouge the Bat went looking for Eggman's secret treasure in his base, she accidentally reactivated E-123 Omega when she released Shadow from a stasis pod. Mistaking Shadow for an Eggman robot and following his new mission, Omega attempted to take out the hedgehog. However, Rouge broke up the fighting long enough to hear both their stories. He soon teamed up with Rouge and Shadow as the power member of Team Dark.
After Omega and his team finally defeat Eggman (Metal Sonic in disguise). Omega then sets about destroying Eggman's Shadow Androids. Later, Omega teams up with the other teams against the Metal Overlord. Omega is last seen during that event with Shadow exchanging looks with him and holding Metal Sonic's deactivated body in his arms.
Journey to the future
Upon being sent to the future by Mephiles the Dark, Shadow and Rouge notice Omega in stand-by mode in Crisis City. Rouge later goes back to the present alone, where she finds Omega in the Tropical Jungle Zone for unknown reasons, though judging by the throngs of Eggman's robots it can believed Omega was trying to destroy them. Rouge programs him to help Shadow out in the future, in which Omega goes into stand-by mode until the point in the future when Shadow is in trouble. He then wakes up in the future and helps Shadow in a fight against Mephiles, and Shadow and Omega go back to the present.
Later on, Mephiles teases Omega by saying that Omega was clearly made to capture Shadow. Angered by this, Omega proceeded to unload his cannons on Mephiles, temporarily defeating him in the process. Omega then reveals somewhat sadly, that he is the one who captured Shadow in the future due to humanity's fear of him. However, it is currently unknown if the erasing of Iblis from history canceled this future event, as it was Shadow whom the world used as a scapegoat for Iblis' destruction.
Assistance against the Marauders
Omega later went to Eggman's city of Metropolis to destroy any robots he could find, but he found the Nocturnus instead. The Marauders had reprogrammed Eggman's old robots to serve them and had sent them to take Omega apart after he had to retreat into a room in north-west Metropolis. The robots were successful, but Shadow was worried about Omega and went in search of him. Shadow then teamed up with Sonic and co. to help find Omega. Omega was found and repaired by Tails, much to the disapproval of Eggman. When Omega saw Eggman, he was going to attack him, but Tails told him that they'd joined forces with Eggman. At first Omega thought Tails had gone mad and suggested destroying him, but Shadow and Rouge confirmed this for Omega. Trusting his friends more than Tails, Omega teams up with Sonic after recharging himself.
Like many other Eggman robots, Omega has a free will and a personality. He thinks other Eggman robots are inferior to him and that he is the most powerful and even refers to himself as the "ultimate E-Series robot." It also seems that Omega lacks any sort of negativity towards Sonic and his friends. Omega is also very prideful and feels embarrassed after he is defeated by less powerful Eggman robots.
Omega's only goals in life are to take revenge on Eggman for disabling him, and destroy the doctor's other robots to prove himself superior. Omega is also fairly loyal and dependable when you're on his good side, a bit of a holdover from his original purpose as a servant.
Omega has a huge array of extremely destructive weapons concealed in his arms, (somewhat similar to that of Grounder), including two rapid-fire machine guns, two flamethrowers, two missile launchers, two powerful energy cannons and likely many more weapons. Omega can also utilize jet boosters built in his back to help accelerate and hover, making him somewhat fast. His walking speed isn't very impressive, though.
He has also displayed considerable physical strength, capable of destroying thick doors and giant blocks of stone with his metal fists. He also has some kind of built in sensor that allows him to locate enemies and other various objects.