The Character You First Fell in Love With:
I seriously got into BtVS as an ensemble show, and liked all the primary characters equally--which at the time I started watching in season three was Buffy, Willow, Xander, and Giles. (Faith, Angel, Oz, Cordy, Wes--all still supporting characters to me, in terms of how I think about the show.) For a long time I thought of BtVS as a "nonslashy" show, and it's usually character fixation that gets me slashy, so love of the ensemble and nonslashiness go together in this early phase. Later, I liked Spike--*liked* him, pretty much in the same way I liked all the others. I don't think it was until he got together with Buffy canonically that I really fell for him, though. My memory could be fuzzy but I always think of Buffy as the catalyst there.
The Character You Never Expected to Love as Much as You Do Now:
I don't think I've fallen for anyone in a surprising way, but I'll note (as I said to Kita earlier today), that the more I pair Spike off with other characters (in my head), the more I fall back on Buffy/Angel as a kind of OTP background ship. Which *really* surprises me. I certainly respect it, and I try to keep Buffy's strong feelings for Angel in mind even when writing noir.
Maybe Xander--he was just one of the guys until I started pairing him with Spike, and ever since then I've loved burrowing into his head, voicing his voice. Sometimes I see people say that when Xander sent Buffy off to kill Angel--as Willow was trying to spell back his soul--they came to despise him. Me, I thought that kicked ass. Not as a positive character trait, but as a dark one, a complicated dramatic facet. I thought: Joss, you fucking *rule*. Xander's treatment of Anya never bothered me either. I mean: whatever. I like to watch people who are fucked up and fucked over and dysfunctional.
The Character Everyone Else Loves That You Don't:
Faith. She has no backstory. Zip. She came onto BtVS, all tortured and twisted, but she didn't grow with the series like many other characters--she didn't grow on me. She had a sporadic presence with her sociopathic arc in *one* season, and then some guest spots on Angel. I don't get the big deal. I *like* her as a quasi-sociopath. Like I've said, I think fucked-up characters are fun, and when I was watching the show in real time, I loved her relationship with the Mayor, loved the whole season and how she fit into it--but to me she'll always be a peripheral character. She's lip-gloss and superficial toughness, and I don't find it interesting to focus the spotlight on her in fiction, though I'm totally happy for people who want to. :>)
(ETA: Although I should note: whenever I get into stretch mode, where I'm reading fiction outside my OTPs, I am happily willing to be seduced into love of a character, no matter how minor. I still remember with amazement that one story about the vampire character Sunday, who was incredibly marginal. But it was an incredibly cool, dark, and fascinating story.)
The Character You Love That Everyone Else Hates:
I love the Mayor--but doesn't everyone love the Mayor? I don't think I have anyone for this category.
The Character You Used to Love but Don't Any Longer:
I worry that this will someday be Spike. But for now I don't have anyone who fits this category--except on Angel, where it's Cordelia.