Sid is a mean, aggressive pre-teen who spends his time torturing small animals. Seth is his lanky little yes-man who hides behind his buddy's bulk when shouting insults. They love to bully total nerds, so living in Science City must be like Christmas for them. Of the two, Sid is clearly the more dominant, pushing Seth around nearly as much as he does his actual targets.
Dreamwave Armada comics continuity
While Sid was occupied trying to kill squirrels with his Dennis the Menace-style slingshot, Seth noticed their arch enemies, nerds, trying to have a good time. As Rad, Carlos and Alexis took off in their spectacular splendiferous balloon-mobile, Sid and Seth saw it as the perfect opportunity to send those three poindexters to an early grave. Using his slingshot, Sid popped the balloons keeping the contraption afloat. As the nerdlingers (well, actually just a dummy) plummeted to their deaths, Sid made a mean-spirited quip as Seth encouraged him to make a break for it, lest the law catch onto their gruesome shenanigans.
Little did they know, their homicidal plot didn't actually kill the paste-eating dorks, but helped them to become best friends with a team of awesome super powered robots from outer space. Take that, you little sociopaths!