From Sonic Retro
The HEXAeco corporation is an energy company of Sonic's world. It appears only in Sonic Heroes, where the third and fourth levels (Grand Metropolis and Power Plant) take place in a futuristic city apparently dominated by HEXAeco.
In Team Sonic's story, "Eggman" flees into the city after his defeat in the Egg Hawk. His robots seem to have occupied Grand Metropolis and are siphoning HEXAeco's energy out of the place, shutting down the iridescant sky-roads and no doubt causing all other sorts of havoc.
The company's logo is a stylised green hexagon, thus hitting both the "hexa" and the "eco" of its nomenclature. It seems to be a not-too-subtle allusion to Texaco, a real-world oil conglomerate.