Ahh, the famous Xelloss-Filia debate. Ever since Slayers pages (specifically character shrines) started poping up around the net, there's been a big debate about wether or not these two actually have feelings for each other, despite their neverending fighting. Before I start I'd like to mention that everything I say from this point on is *my* opinion and I am entitled to say whatever I want since its my site ^_^ (note: this might seem a little unorganized because I had no time to do any major changes to it. When I have more time I'll change it)
Slayers is known for their "opposites attract" pairings. Such is the case of Xelloss and Filia. They are very different in a lot of ways, the most notable is obviously the fact that he's a Mazoku and she's a Ryuuzoku. Ever since anyone can remember, Mazokus and Ryuuzokus have been at war with each other over power of the worlds. Their battle is neverending and this makes for some great and very funny scenes between them.
Some say that Xelloss just teases Filia to hide his 'true' feelings and vice versa. And many use to back up their statement the scene at the end of Try when Filia finally accepts to cimbine her powers with him: "Now that's the dragon I know" (note: In the dubbed version Xel says "Now that's the dragon I love." This makes for some confusion.) and Filia smiles at him. Other scenes are when they get paired in the orb episode and both complain and say the exact same thing at the same time. This may show some similarities between the two personality wise but nothing that may signify something 'more'.
Theirs is a 'I hate you but I can't to anything about it given the circumstaces' relationship but both manage to work around that... well, not really. In fact its quite the opposite: they take every chance they can to piss, belittle or humiliate each other, no matter what and take great pleasure in doing it, too. Of course, Filia is much more hypocritical about it but thats another thing... ^_^ But despite everything, they both have a 'certain' amount of respect for each other and, in the end, are able to put aside their differences and work together (but just barely).
So the last thing I'm going to say is this and this is my opinion on the Xel/Filia couple in general: I am not a Xel/Filia fan. I do acknowledge the fact that many see them as a couple and that there are many good and valid arguments for them being portrayed as one but I simply do not see Xelloss with Filia... or anybody for that matter! Thats just how it is for me. So no flames :P