Everytime Frisk resets, Chara is able to manipulate Everyone's memories [data] but not for Flowey and Sans, they use this power so Frisk will believe they are both siblings, Chara erased their image from who knew them. And told frisk that breaking the barrier is the end of their existence, which make Frisk continuously reset towards 'The end' leaving Sans' Friendship.
When Frisk decides they wont reset anymore, Chara take things into their own hands, Sans is sure that Chara is not who they say they are, but Frisk doesn't believe him.
In the beginning, Frisk was about to reset. Sans started to tell if Frisk was bored already, reminding them that they didn't go to Hotland yet thinking Frisk was being impatient. Sans doesn't get bothered when Frisk reset anymore, Then he says 'See you in the next run' knowing Frisk might reset, after responding that to Frisk, He teleported away. Frisk started to call Sans 'Liar'
The Last Reset
The Prince, Asriel
Those are stories for the future. They would be removed in this list if they are currently work on.