My sister used to watch it, and I never really cared for it. I was never a fan of vampire shows. Then I caught a glimpse of Nicholas Brendon and I was all, "Wow, who's the hottie?" It was Becoming Part 2. I remember it was the scene where Xander lied to Buffy. My sister was enraged, but I was intrigued. I love flawed characters. It still didn't capture my full interest though. Later, I'd catch scenes here and there, mostly the Xander ones. The first episode I watched from start to end was the Zeppo. And I fell in love with Xander. My sister was already hooked on Spander ( Xander/Spike) fanfiction and I asked her to print me an angsty Xander fanfic. She gave me all the Spander fanfic she had printed.
I devoured them! The funny thing was, I didn't know what Spike looked like and I was too lazy looking it up on the net. I was mostly interested in Xander. But I had very high expectations about Spike's looks, almost every fic described him as a sex god. When my sister showed him to me, I was really disappointed, I thought he looked average and that Xander was better looking.
I kept reading Spander fics until S4 started airing in my country. It was the first official season I watched. The Freshman didn't have Xander, and my sister told me, according to fics, Xander wouldn't show up until episodes later when Willow and Tara fell in love. I was really upset!!! But suddenly he appeared in the Bronze, I fell off my chair squealing and my sister jumped up and down with happiness, honestly I think we were happier than Buffy when he showed up. 😀
Watching the show regularly, I fell in love with all the characters instead of just Xander and to a smaller extent Spike. Sigh. Good times.
So, I guess I fell in love with Buffy through Spander fanfic first and then the show.