Board Thread:Questions and Answers/@comment-25855697-20170105010828/@comment-27701762-20170105015943

Both post-Genocide and post-TP, Frisk (i.e. the protagonist) behaves the exact same way as if the game is booted up for the very first time. They do not tell Asgore they've been killed before, they don't give Sans any weird looks when he says that they haven't died (unless of course, you have died since restarting the game), they react to Sans as though they don't know he's going to tell them to turn around, etc., etc. Presumably, if Toby had intended for Frisk to remember this stuff, there would be some indication in their behavior which would provide a clue. But no such clue exists. The conclusion should be, then, that Frisk does not remember anything once the world is completely reset after a Genocide or True Pacifist run.