Undertale fics that lean into the whole 'what if Flowey had a friend he couldn't just reset over' dynamic tend to bring out some interesting stuff. A lot of it circles around Papyrus being that immovable object of relentless positivity and friendship poking holes in Flowey's detached nihilism. You get a ton of AUs where Papyrus, through sheer stubborn kindness, becomes the one character Flowey can't actually bring himself to kill, which creates this weird, possessive form of 'care' from Flowey's side.
Then there's the more common hurt/comfort angle, where Flowey is just... a mess after the True Pacifist route, and Papyrus decides to adopt him. It plays with the idea of Papyrus's innocence acting as a shield against Flowey's bitterness, and the tension comes from whether Flowey will corrupt that or learn to protect it. Sometimes writers flip it and explore Papyrus as secretly far more perceptive than he lets on, seeing the lost child in the flower and choosing to engage with that, not the monster.
Less talked about but still around are darker takes where Flowey's manipulation is more overt, and Papyrus's trust becomes a tragic flaw, or stories where they're both post-genocide survivors trying to rebuild something in a reset world. The contrast in their worldviews makes for endless 'odd couple' scenarios, whether played for laughs or deep drama.