During the fifth season episode "Dr. Urkel and Mr. Cool," Urkel devised the ultimate plan to win Laura's heart: transforming his DNA using a serum called "Cool Juice" to suppress his "nerd" genes and bring out his "cool" genes. This resulted in the alter ego known as Stefan Urquelle, played by White in more casual attire and with a smoother delivery. Initially, Laura is enamored with Stefan, but asks that he turn back into Steve when Stefan's self-centered, narcissistic attitude comes out.
Steve later improved the formula to limit the effects it had on his new personality, and re-dubbed the formula "Boss Sauce." He also invented a "transformation chamber," which allowed him to turn into Stefan for extended periods of time (as well as avoiding developing rashes in "personal areas"). He changed into Stefan several times—even while dating Myra—but some circumstance caused him to change back into Steve each time. In the Season 6 two parter episode: To Be or Not To Be, Steve was stuck as Stefan for a while, after Carl was accidentally transformed into Carl Urkel, due to Myra's tampering with the transformation chamber to sabotage it and preventing Stefan. Late in the sixth season, Steve transformed into Stefan as part of an inventors competition at Walt Disney World; however, Laura sabotaged the transformation chamber to prevent him from turning back into Steve. During his extended stint as Stefan, he proposed to Laura in front of Cinderella Castle. Laura accepted, but their engagement was broken off when Myra appealed to Stefan and Laura revealed her sabotage. While everyone liked Stefan, Myra disliked him immensely, believing he was a joke to her and thought her "Steviekins" is perfect the way he is.
In the seventh season finale "Send in the Clones", Steve created a cloning machine. He came to believe it did not work though it did create another Steve. To clear up the situation, Laura proposed that one of the Steves be turned into the suave Stefan.
The permanent Stefan made several more appearances throughout the series and proposed to Laura again before the series finale. After weighing her choices in the flashback episode "Pop Goes the Question," Laura chose Steve over Stefan. He left and never reappeared again. It could be assumed that Stefan accepted their relationship and moved on with his life, after he learned from his mistake in the episode "Crazy For You" when he teamed up with Myra to stop their date.