Originally, Zack and Slater were written as bitter rivals for Kelly's romantic affections. Off-screen, Mario Lopez and Mark-Paul Gosselaar were (and continue to be) very close friends. The writers, sensing their obvious chemistry, had the characters re-written as best friends, and they remained that way for the rest of the show.
At different points throughout the run of the series, Mark-Paul Gosselaar dated all three of his female co-stars. Elizabeth Berkley was first, then Lark Voorhies, and then Tiffani Thiessen--creating some tension among all of them, at which point, the executive producer had to sit down and talk to them because there were times when Lark, Elizabeth, and Tiffani weren't speaking to each other. He also dated one of the Creel sisters.
Elizabeth Berkley originally auditioned for the role of Kelly Kapowski. Although she didn't receive the part of Kelly, producers liked her so much they wrote the part of Jessie Spano for her.
Although Kelly was Zack's girlfriend for the show, Lark Voorhies was Mark-Paul Gosselaar's real-life girlfriend for three years, including Saved by the Bell: Hawaiian Style (1992), according to several interviews by Gosselaar.