The story, Cat Shadows by Prospero Hibiki, is an AU starting from just after Halloween in S2. And no, it's not yet another Xander's costume schtick.
The general premise is that Xander does some research post-Hyena and post-Soldier possession and finds out he might be more susceptible to future possession attempts. So he does something about it by taking on a Cat Spirit to block any potential possession in the future.
It's probably one of those few Xander-gets-a-power stories and Xander is perfectly okay with it (at least at first) that I truly adore. 16-year-old Xander is soooooo 16-year-old Xander in this that you could actually picture this happening if BtVS chose to go in this direction.
If there's any complaint, the language inside Xander's head is a little more formal than you'd expect (especially since it is told from a first-person point of view), but just seeing Xander enjoy his new abilities (as well as suffer some related drawbacks *because* of those new abilities and the clever ways he learns to get around it) really lifts this fic. Toss in the fluffy fun and humor, and it's too charming not to enjoy.
The pairing is B/X (although you do see the B/A relationship). If that bothers you, please give this story a try anyway. It's not hot-and-heavy B/X and it takes some unusual forms in some laugh-out-loud moments before Buffy figures out just who her "patrol buddy" is. (No really, you can't blame Buffy for not figuring it out sooner.) The relationships between all the Scoobs (even the romance stuff) is perfectly in tune with the fluffier fun of S1 and the beginning of S2 BtVS.
Even more extraordinary, Angel comes off as pretty good here (a rarity in High School-set B/X fics) even though we're seeing the Brooding One through Xander's green-tinged (and I may mean that literally) eyes.
If you're one of those that tend to shy away from Xander-gets-a-power fics (and I know I am) this is really the exception you have to read.
Go. Read. This fic is like a warm fuzzy blanky on a cold winter day. It's minimal angst, lots of laughs, and a nice look at the Scoobs and Angel before they grew up and had to carry the weight of the world on their shoulders.
I'll be honest, Cat Shadows is on my list of "feel good" fics, which includes Wisdom of the Fool, Folly of the Wise by Pete Milan, one of the more breath-taking, insano-making post-S4 AU's you'll ever read (This one is W/X).
Another feel good series to check out is bastardsnow's Coffee Series which manages the impossible: fluffy romp, post-'Chosen', B/W/X friendship, and a series of clever cross-overs that aren't too heavy-handed. Read them in order of posting date on the archive page rather than order of appearnce. I admit, I have to be in the mood to get fluffed to read it, but when I need fluff, I reach for Coffee. *smooches Mark for writing these stories since I needed fluff last week* Someone kick him so he'll start posting his fic on LJ.
You know? I'm beginning to think that F/X is not a pairing that lends itself to fluffy. I can't think of a single fluffy F/X story.
Oh, new Living History part will be up tonight: Xander and Faith have a last chance to talk to generations as-yet unborn. It's not pretty sight.