Sigma is actually Eggman from the Sonic universe after crossing the streams and having his body fried in the process for plot reasons why not, so they had to build some of his body but fucked up and put his brain into El Gigantor robot body and made him FUCKING BARA because why the fuck not.

TL;DR SIGMA IS CYBORG BARA EGGMAN WITHOUT THE MOUSTACHE!!! HOLY SHIT!!!