What Wikipedia Was Made For

1: "Blah blah blah I used to live next to Urkel."
2: "Hey, what was the deal with that Stephan Urquelle character? I remember Urkel had this alter ego, but I never found out how they explained it."
3: "I feel like this is one of the few questions wikipedia was designed to answer..."

So we looked it up, and here it is. I must say, I'm very impressed with the complexity of the Urkel mythology. I had no idea it was so developed and systematized (ie "boss sauce" and "Bruce juice").

Stefan Urquelle is Steve Urkel's alter ego, also portrayed by Jaleel White. While Urkel is clumsy and unpopular with people around him, Urquelle is popular and suave, especially with women.

Steve created a transformation chamber which allowed him to turn into Urquelle whenever he wanted to; said chamber runs on a liquid known as "Boss Sauce". Steve makes the transformation to Stefan numerous times mainly to woo Laura Winslow, who, as with other women encountered by Stefan, is enamored with the persona. In his premiere, however, he was very narcissistic, cold-hearted, Gaston-ish, and shallow, which quickly turned off Laura.

The effects of the transformation chamber are only temporary; Stefan will change back into Steve after a certain amount of time (originally a week). As the show went on, Steve enhanced the "Boss Sauce" which eventually stopped the change-over from Stefan back into Steve. Stefan could only change back into Steve by drinking an antidote or chewing a tablet. He is also able to alter the nature of his "Boss Sauce" to induce other personalities, such as morphing himself into a martial arts master using hair samples from legendary martial artist Bruce Lee (he referred to this altered "Boss Sauce" as "Bruce Juice").

Later on in the series, very near to the final half year of the series in fact, Steve creates a cloning machine which allows Stefan to become a real person (though it was actually Laura who put the other clone into the chamber), and throughout the rest of the series, he was Laura's boyfriend, and he never had to change back into Steve.

In one episode though, the Winslows go to Paris, and Stefan decides to stay and work in fashion, though it didn't end the relationship. A few episodes after, Stefan couldn't resist the fact of leaving Laura, and decides to move back to Chicago where he sees her from stage at a New Edition concert.

The two were together again until, one of Laura's former boyfriend, Curtis returns to the show, where he is told from Eddie that Laura still loves him (though this was only a prank to get back at Laura). Curtis tries to get her back to him, but when he realises Stefan is her boyfriend, he gives him a warning that if he ever breaks Laura's heart, he'll get what he deserves. Stefan, unfortunately, believes that will never happen.

Near the series' finale, Stefan proposes to Laura, but at the same time, Steve does the same. In the end, Laura chooses Steve over Stefan finally ending their long-distance relationship (that went on until Stefan was finally a real person). To that, Stefan leaves, and the rest of the series goes on without him in the last episode.