Undertale: Rotulus is an AU focused on one small change. Asriel's Survival during the plan.
Here, Asriel managed to flee fast enough to return. He almost died, but in a miracle...
"the young prince stayed determined. The underground saw this, saw their princes new found powers. Saw how even after death, their human sibling remained conscious. Saw how the two co-existed in the same body. And realized this was no normal act.
Chara's sacrifice was not a tragedy, but a gift, the angel has made itself present by the two kids lives... And now resides within them."
This interpretation of events turns the underground into a theocracy focused in the two children and the prophecy.
Another change is Asriel begging Asgore to not retaliate, and since his son was alive to forgive, and was now belived to be the representation of a god, it's hard to ignore those pleas. (Chara was not really happy at that,) meaning the underground welcomed humans, some humans were good, others...not so much, one wasn't satisfied with staying he could do so much more, so he killed King Asgore and left quick enough for no one to notice, rendering reloading useless... Asriel was an older teenager here.
Now, you saw the drawing, how is Flowey here?
Sequence of events
Asriel being Alive meant his parents growing older, and him eventually inhereting the throne. When he did that, he hired and tasked the royal scientist to both reaserch souls from the humans who died of old age.. And with creating a new body for his best friend.
Chara was very specific with their body. Most were suggestions, but there was a direct MUST in their list.
"IT CANT BE HUMANOID."
They were known for liking golden flowers... So they used one to make their vessel.
The flower was found a few weeks later inside the vents, repeating words inside of it...
Thats All I have about these three characters, soon I will make a post exclusive tk chara since this one was more centered around Asriel and Flowey.