Dexter Thornhill, played by Robert Laughlin, is Steve's rival similar to Cassie Lynn Nubbles was to Laura Winslow. He was a Caucasian nerd who went out of his way to outdo him in everything. His only appearance was in Season 5 episode "Presumed Urkel," where Steve is accused of blowing up the school's science lab just before a competition that he and Thornhill were to take part in. In the upcoming student trial where Steve is to be expelled from school if found guilty, Thornhill is the prosecutor and calls in the science teacher, Mr. Sweeney, and Carl. However, the former states while the last person he saw in his room before the explosion was Steve, he never actually saw the explosion and thus, does not know who really caused it. Carl states that while Steve has done catastrophic damage in his house numerous times, he never denied causing any of them like he did with the lab explosion and had saved his life before.

After calling Steve and the school custodian, Mr. Looney, who was in the room with Steve right before the explosion, Laura, acting as the defense, sees how anxious Thornhill is in trying to get Steve expelled and calls him to the stand. With a blacklight lantern, she was able to prove his guilt by making him reveal chemicals on his hands. Dexter loses it and confesses to his crime, stating that he caused the explosion by adding extra explosive chemicals in Urkel's graffiti solvent because he deserved to have won 1st prize in the science fair years ago. At the end of his confession he exclaims "Darn you Urkel darn you to heck!!". He was tired of staying up into the early morning hours, while Steve only glances at a page and gets straight-As and condemns him for being better than him. After the charges against Steve are dropped, Dexter was expelled for his crime in destroying the lab and is never seen or heard from again.