I honestly wasn't sure about that pairing at first—Caelus and Sunday feel like they inhabit such different worlds in 'Honkai: Star Rail'. The stories that really got me are the ones that lean into that cosmic/divine versus grounded traveler dynamic. There's a long one called 'Quiet Hour in the Dreamscape' that treats Sunday's whole vibe with such eerie reverence, while Caelus is just this pragmatic, slightly confused anchor. It's less about romance and more about two utterly different forms of existence trying to understand each other. The tension comes from whether their natures are fundamentally incompatible.
Some shorter fics go the enemies-to-allies route, which can be fun if the writing is sharp, but they often fall into cliché. The best avoid making Sunday a mustache-twirling villain; he's too interesting for that. I skip anything where Caelus 'fixes' him—that misses the point entirely. The appeal is the push-pull, the strange beauty of their contrasting aesthetics, not fixing one to suit the other. I've found the most compelling takes on Archive of Our Own by filtering for the 'metaphysical' and 'character study' tags.