I'm not too sure how specific you want me to be, but my favorite characterization of them is as an incredibly mean-spirited sitcom -- she never falls in love with him and doesn't bother trying to hide it most of the time. Belos just has to pretend it doesn't bother him. Oh sweetheart, you set my bed on fire again <3 you're so considerate; you know how cold I get <3. On a certain level, Luz realizes the real way to win against him is not to play-- to just grey rock him entirely -- but it takes a long, long time for her to be able to do it both because of her own emotionality and also because of how he escalates.

One thing I thought about but didn't put into "the great war" was her teaching him more modern slang -- I like Belos would be a big fan of saying "large if factual" after Luz says something is "big if true." He DEFINITELY snoops on Luz's fanfic writing/reading habits (see my vore tweet). This is Belos on Reddit: https://www.youtube.com/watch?v=pfLCm24I83w

My spiciest philuz HC is that Luz does that zoomer "oh you're calling me a [insert slur]?" thing and Belos, not wanting to admit that he's playing with the Bigotry base game and hasn't gotten any of the expansions, just goes "sure."

If we're talking about her meeting him as Philip first, I think she'd be easy pickings -- thinking of Luz carving glyphs into her skin to impress him, because you know she'd at least try.