Eggman is a short-tempered, raucous, pompous, cruel, evil genius who is constantly planning to take over the world. His primary motivation throughout most games is his psychotic belief of self-entitlement. Arrogant and narcissistic, he constructs countless technological wonders in his own image to fuel his massive ego, usually reinforced by his goal of finally constructing the Eggman Empire, and doesn't care where it starts. His narcissism is such that when he first discovered the Phantom Ruby, what it showed as his deepest desire was Eggmanland complete with various labels of his names and statues to himself, and was saddened when the illusion ended.

Although Eggman is a villain, he helps Sonic when the world is at stake; though the reason is always because he knows there would be nothing left for him to conquer if the planet was destroyed. In some of his incarnations, he actually displays a begrudging respect of his arch-enemy, Sonic.

Despite his intelligence, Eggman can be very immature, throwing temper tantrums when Sonic manages to wreck into his plans. 

... Y'all all know that Eggman should sound like. Like come on.