My personal Deltarune head cannon is that the darkners don't just exist for lightners, they get their power from the love of lightners too.
This is why the bosses (except chapter 4s) get progressively stronger as you go on. Each object that represents them gets progressively more loved as the game goes on.
King is a card that's been abandoned for a long time
Queen is a laptop that's been used but never really loved
Tenna is a TV that, while everyone doesn't use him much anymore, is still a beloved centerpiece of the house.
And Flowery is a flower that's been lovingly cared for and kept alive for years.
If love for the darkners themselves and not just them in object form counts, this also explains why ralsei gets more and more powerful throughout the game despite not killing anyone. Because Kris and Susie love him.
If you assume the shadow crystals are their own thing independent of love, then the two who don't use them (Gerson and maybe Pink) have reasons they're so powerful too. Pink is a super boss because the ghost loves her. Maybe in the real world it was just as a body, but everyone else was just loved as objects too. No other darkner has a bond like that with a lightner (except maybe ralsei depending on what theory you like best). And Gerson is strong because, well, he's a grave of someone beloved that many people have mourned from, which is way deeper a connection then you have to something like a TV. Another theory is that the love people have for Gerson carried over into his darkner form, but idk how darkner ghosts work.
