I think the thing I like best about how you write these is how you keep Xellos both mysterious and don't go too far with how he keeps teasing Filia. I don't ever expect him to admit to what he does, because he's a mazoku and I can't figure romance into the equasion. But Filia can't help but question it because she /can/ feel that sort of attraction even if she doesn't want to admit it. You're good at creating that tension.