.: Don't Leave :.

 
"Even if it means i have to kill you a million times, yah not leaving in my watch"

 i could imagine in genocide route sans didn't care that much so he didn't even bother stopping frisk/chara but in pacifist route i could imagine he would fight you thinking that once the barrier is opened frisk would leave and and forget about UF sans and he doesn't want that he used to protected her as best as he can but after they started getting throw the core sans noticed and he didn't want that to happen so he disappeared for a while until frisk got to the judgement hall and sans was there waiting for her

then the battle began chara wanted to help frisk by not letting her die 
((you know when you battle asriel and it says "but it refused"))

and i could imagine chara being an angel in this au and the whole genocide route was cause he thought it was best since everyone in the underground is well "monsters" and he killed every monster but when the got to the barrier frisk cries not wanting to hurt anyone and then chara offers her to reset gave her luck trying to befriend all of the mosters 

i imagine chara in this Au well is an angel but his younger then frisk and always offers help and protects frisk and flowey as best as he could after frisk befriend everyone he tries to watch over others his a sweetheart but he isn't afraid to attack if needed

welp hope you like it ^^