This is how all my deltarune theories play off each other into one cohesive story

first and foremost i wanna bring up the theory of the darkners representing various RPG tropes played straight. Ralsei is clearly the mage though if thats too plain they could be the call to adventure since it seems like they were the one who brought kris and susie into the dark realm. Lancer is obviously the lancer. not only is he the trope by name but he fits very well as the protagonists foil. he is like kris in every opposite way. Jevil is the secret boss played straight and the spades king is the villain who does terrible shit for the sake of being evil.

Now with the first theory out of the way, Its reasonable to think that every lightener has a darkener counterpart. afterall, everyone has a shadow. Ralsei would be asriels, his name is an anagram and his tutorial is a twist on floweys tutorial. And even humans can have darkner counterparts. Kris’s darkener counterpart would be chara. 

Third theory of chara being a darkener originally is that they themselves claim to embody the mechanics of RPG games. the drive to kill and get stronger no matter the cost. And what better way to do that than to steal and merge with the strongest fighters. in genocide they steal frisks body because frisk shows them strength through violence and in true ending, they save frisk and asriel because frisk shows them strength in bonds. It would explain why sans tells you to keep pretending to be human in genocide rout. As for sans…

I think sans is a darkener too. I said before that lighteners and darkeners have counterparts it would likely be gasters counterpart. Gaster could very well have used science to pull a darkener into the light world. Since hes confirmed to be older than papyrus, it seems like it would be easy for pap to think of him as a brother. now which trope would he as a darkener represent? He attacks first and can idle to prevent you from attacking as well as fast travel and has interracted with most if not all NPCS. Sans is the protagonist.

Penultimate theory; gaster rebooted the universe when he fell into his creation (which is probably not the core.) His goners are alive and well in delta rune. I think this is evidence of being pre scratch. one is in kris’s class, one is in the library and one is stuck in traffic. i think that they were present when gaster fell into his creation and that caused a massive temporal shift which rebooted the universe and stuck the soon to be goners into the state theyre in during the events of undertale. We know that hard resets are entirely possible, it happens after genocide and true endings of undertale so a hard universal reset could be done if gaster was experimenting with determination. Now here comes my final theory

With all these theories in mind, they culminate to this final one. Darkeners are slightly resistant to being reset. Not completely but partially. It would explain why sans knows what a sun is. why he knows when youve reset. How he can recognize chara. and how chara could continue to possess frisk despite hard resets. Sans as a darkener would probably be aware of the first instance of the universe as well as the dark kingdom and considering where he was, there is an absolute zero chance of him ever making it back home.

These are just theories built on theories with the limited knowledge that we have. Some of the reasons why i theorize this will likely be debunked in future installments of delta rune, but i think that it would make a compelling story of cat and mouse between sans and chara if it was true.