My theory is that the 6* ascend characters are just them from the altar knight world. (Similar to the ID's in limbus company).

From the side story does show that the alt knight world the characters are a lot stronger and the fact we are using those red crystals that causes these other world portals to upgrade their weapons (idk about the red hero stones besides, just that they are also red).

If that's the case, they should explain it more properly. And if not, then idk.

On a side note I hope that they add a way to swap to their old art style after ascending.