Well, the current way the AU used OCs are as Figments, manifestation of a Fan's INSPIRATION. They have a magical link where they passively consume magic from their creator, so when the creator dies, they die.
My idea is that when you were the still LuxrayBlast and the captain of the Guard, you died fighting a genocidal Surfacer (I don't have a better name) and your conscious was transferred to Tinsle due to your DETERMINATION while the other OCs faded away. So now you live on as a SOULless Figment, like Asriel living on as a SOULless flower.
I'm tinkering with the roles to see what would fit best.
I figured due to the backstory I'm planning for the AU, the "original" Chara & Asriel roles should be in an extended role. With two other people as the "new" Chara & Asriel (so technically there's 2 Charas and Asriels).
But it means I need 2 more people to fill in Papyrus/Undyne/"new" Chara (I'll be in one of those three), which would make it more complicated. So if I can't make it work I can still make you Papyrus.