I have read so many fics where Willow was portrayed as anti-Spuffy and it felt strange to me. Of all the core four, Willow is the logical choice for a supportive friend. She doesn't hate Spike and she had supported all Buffy's previous relationships, so why not Spuffy? Someone once told me that Willow's reaction when Tara had told her about Spike and Buffy in Seeing Red is the answer. But Willow's shock was clearly because Buffy had made it clear since the beginning that she will never have anything to do with Spike in that department. So her shock was mostly about Buffy choosing to be with Spike rather than the relationship itself.

If you'd like to see a good example of Willow's reaction to hearing about a Buffy and Spike relationship of some kind, then watch her in "Intervention."  Willow is very supportive.  Well, the most supportive.  She's not disgusted by it, just confused.  I think of all the Scoobies, Willow was the biggest Spike fan.  No matter what it was, she would always try to encourage him or be supportive (for some reason).  And Xander (who I think was a closet Spike fan), if told about a Spike/Buffy relationship, would have bitched about it for all of twenty-four hours, and then gotten over it. 

As for Willow's reaction in SR, well I don't think she was really too focused on Buffy to notice.  She had just gotten Tara back, and before that, there was always something going on with her.  She was more like, "Nah, Spike and Buffy?  Never happen" as opposed to "Spike and Buffy? *gasp* Buffy would never degrade herself in such a way!"

Damn it...that was me, Scarlet.

Willow's reaction in SR always seemed to me like she was more upset that she was the last to know than that Buffy and Spike were together (or had been, at that point), which I think is very in character for Willow.

I think fics that portray her as anti-Spike or anti-Spuffy are generally based on the fact that Spike has tried to kill her more than anyone else (other than Buffy). Also, if the theme of the fic is "Spuffy=right, Scoobies=wrong" then it messes things up if Willow is supportive. Personally, I like my fics a little less black and white than that, but it seems to be a popular fanfic trope.

Willow was always the most supportive of him despite that fact. I found the always had a slight friendship and respect for each other. In Lover's Walk and Intiative, they even do end up quasi-comforting one another.

I don't think Willow would be anti-Spuffy at all. Maybe a little worried at first, over what happened with Angel, but I think overall she would be supportive.

What Eowyn said. LOL

I don't think that canon Willow would have been anti-Spuffy.  In "Intervention," she takes Buffy(bot) and tries to understand where Buffy(bot)'s coming from:

BUFFYBOT: I had sex with Spike. (Willow winces again) I'm sorry if it bothers you. You're my best friend.
WILLOW: I-I am. And I, I always will be, no matter what you do. I, I'm just trying to figure out why this happened, and I, I think with ... your mom and everything ... everyone was being all sympathetic, and, and making you feel weak. A-and Spike wasn't like that. So, just this one time, you just ... did something kinda ... crazy.
BUFFYBOT: (shakes head) It wasn't one time. It was lots of times. And lots of different ways. I could make sketches.
WILLOW: (disgusted) No! Buffy, there is something seriously wrong here! I ... (the bot looks confused) Okay, yeah, you've been with a vampire before, but Angel had a soul.
BUFFYBOT: Angel's lame. His hair grows straight up, and he's bloody stupid.
WILLOW: (confused) Okay ... look, I just wanna help you. Let me help you.

Willow's a little wigged out, but she's definitely not condemning Buffy(bot).  No, she's not, "Woohoo!  Buffy boned Spike!," but she's not, "I'm disgusted because Buffy's boning an evil, soulless thing."  She's trying to understand what's going on because she's Buffy's friend, and she wants to be there for Buffy.

In "Seeing Red," Willow has pretty much the same reaction. 

WILLOW: Yeah, I'm not really worried about her going up against Warren and the others. I know this is going to sound crazy, but ... I think something might be going on. With Spike and Buffy. (Pause) I mean, she looked so hurt when she saw him with Anya. I think maybe-
TARA: They've been sleeping together.
WILLOW: (laughs) No. I wouldn't go that far.
TARA: No, I mean she told me they've been sleeping together.

Willow sits up to stare at Tara.

WILLOW: Sleeping together? You mean like the naked kind of together?
TARA: (sighs) I'm sorry I didn't say anything, but I, I promised her I wouldn't.
WILLOW: (shocked) Does everybody else know? Am I the only one she didn't-
TARA: God, no. She, she didn't even mean to tell me, it just came out.
WILLOW: (still shocked) How could she hide something like this from me?
TARA: I think she was afraid of the look you'd get on your face. (Willow staring at her) Kinda like the one you're wearing now.
WILLOW: Oh, no, I'm not ... I'm just trying to understand.
TARA: So is she.
WILLOW: Wow, she probably really needs someone to talk to.
TARA: Probably.

Willow just wants to understand what's going on with Buffy, and she's hurt that Buffy didn't tell her (not that everybody had been all with the sharing at the time anyway :-P )

Based on these reactions, I think that, of Xander, Giles, and Willow, she would be the one most likely to not react badly to Spuffy.  I'm not saying she'd hop right on the Spuffy bandwagon, like maybe Dawn would (pre-SR), but she wouldn't be completely unsupportive.  I always got the feeling that Willow never minded having Spike around.  She tries to stop him from staking himself, she bonds with him a little when he can't bite her, she seems to be comfortable around him when she's fixing the Buffybot in Bargaining I, etc.  Maybe something inside Willow recognizes Spike's inner nerd as kindred, or perhaps she's more accepting of unconventional relationships, as she knows what it is to be in one.  Now, I'm not saying that a writer couldn't write certain events to make Willow's disapproval believable, but it would probably take a lot to convince me.

always got the feeling that Willow never minded having Spike around. She tries to stop him from staking himself, she bonds with him a little when he can't bite her, she seems to be comfortable around him when she's fixing the Buffybot in Bargaining I, etc. Maybe something inside Willow recognizes Spike's inner nerd as kindred, or perhaps she's more accepting of unconventional relationships, as she knows what it is to be in one.
Those examples aside, I always recall in "The Replacement" when Spike's looking for stuff to spruce up his crypt, and he finds that lamp, and Willow's all like, "It's very pretty."  Which was kind.  It was an unnecessary encouragement.  It was sweet and endearing, and I honestly don't picture Willow (or recall her for that matter) doing more of the same to Faith, Anya, or Cordelia.  Realizing this also makes it funny :P

  Also, he was the only one to fully acknowledge that Willow was still hurting over Oz's departure.  Methinks they may have had a heart to heart at some point in s4...

I always recall in "The Replacement" when Spike's looking for stuff to spruce up his crypt, and he finds that lamp, and Willow's all like, "It's very pretty." Which was kind. It was an unnecessary encouragement. It was sweet and endearing, and I honestly don't picture Willow (or recall her for that matter) doing more of the same to Faith, Anya, or Cordelia. Realizing this also makes it funny

Awww, I forgot all about the pretty lamp.  They're so cute when they interact.  Two of my favorite scenes are Spike and Willow in Lovers Walk and when Spike finds out he's impotent.  He's all encouraging when he says he'd bite her and thought about it when he was shoving a bottle in her face.  It's so cute and wrong at the same time :-)

And, no, Willow did not like Anya, Cordelia, and most of all, Faith.  I can see Willow not liking Cordelia.  She picked on Willow her entire academic life, and then ends up smooching with Xander, Willow's best friend and long-time crush.  Anya--well, Anya's Anya, and those two never rubbed the right way.  Willow hated Faith with a passion.  She almost stole Willow's best friend when she got buddy-buddy with Buffy, and then she tried to frame Buffy for murder.  I don't know about anyone else, but I never take kindly to people who are mean to my friends.  Willow was never outwardly mean to Spike unless he was trying to kill her; and, even then, she was trying to protect herself. 

Now, I'm not saying that a writer couldn't write certain events to make Willow's disapproval believable, but it would probably take a lot to convince me.

I think one of the great things about the complex characters that Joss created is that a good writer can find whatever characteristics  she needs or wants within any one of them, and then spin it in whatever way the story requires. They all had their flashes of unselfish goodness and selfish badness.  You can just pick the "bad" characteristic that you want to explore and expand on it. Willow's need to control, Xander's jealousy, Spike's impulsivity and bad judgment, Buffy's self-centeredness, Dawn's whining, etc. etc.

Having said that, I do think Willow would have been likely to be semi-supportive - both because she sees herself as that kind of friend, and because she has a small soft spot for Spike.  That's how I generally write her, anyway. :)

That was me. Didn't realize I'd been logged out.

It was sweet and endearing, and I honestly don't picture Willow (or recall her for that matter) doing more of the same to Faith, Anya, or Cordelia.
Maybe the reason for that is out of jealousy? The women were competition; Spike wasn't. I mean, she definitely had a reason to hate Cordelia all on her own, but it's interesting that she's accepting of all the men in Buffy's life (Angel and Riley as well as Spike), but dislikes all the women in Xander's life. The Faith jealousy was two-fold, I think - as IB pointed out, she was jealous that Buffy found another Slayer to hang out with, and Willow felt like she was being pushed to second place and losing her best friend status. Then, on top of that, Xander losing his virginity to Faith, and thinking they have some kind of connection. And her issues with Anya were pretty obvious - even if she didn't want Xander for herself by that point, I think she was still very possessive of him.

Also, he was the only one to fully acknowledge that Willow was still hurting over Oz's departure. Methinks they may have had a heart to heart at some point in s4...
Nah, I always took that as just another of Spike's insights, a demonstration of his ability to read people, the way he assessed Buffy and Angel's relationship in "Lover's Walk" or the way he was able to drive the Scoobies apart in "The Yoko Factor." Never got any indication that they'd talked about it, other than Willow's hints at being unloved in "The Initiative."

I agree that Willow was jealous of Faith because she took Buffy and Xander from her in a way. I don't agree, though, that jealousy was the reason she disliked Cordelia and Anya. Cordelia was disliked by Willow before she became Xander's girlfriend. As for Anya, I think the bad hestory between the two of them played a role. Anya tricked Willow to do dark magic, Anya was ruining Willow's pretence as Vampire Willow resulting to Willow almost dying because of Anya. Ever since this episode, Willow couldn't help not trusting Anya again, but with time she was able to accept her as Xander's choice of a girlfriend. She was saddened that Xander left Anya at the altar and she greeted Anya with open arms when she returned. So I think Willow's reasons for disliking Anya were more about them two not clicking together than Xander.