Mal

This happens regardless of Toriel's fate in the game. If you kill her, but not Papyrus, he still comments on this promise.

And here's the thing. He has to make the decision of trusting this woman he is fond of, but ultimately doesn't know beyond what they've shared together.

And Toriel had to make the decision of trusting him too. She is convinved that the world outside the Ruins will be hostile, because she knows what's happened before. And it fucking hurts to think it could happen again. And yeah I've heard all about it how she "rejected" her duty and didn't make the right choice back then. I think Toriel is somewhat aware of that. Her moving to the Ruins was a choice she made while grieving too, but it is a decsion that goes according to her integrity as well.

ANYWAYS I'm getting sidetracked. What I mean with this post is that the implications of trusting someone else with something that is so personal, because you know your limitations (because of flaws or any other cause), and having that person receive this in a good way is so important.

And while Sans is not the best at actively doing that job, it is still remarkable how much this promise is on his mind.

The VERY LAST THING he does before his No Mery fight is apologizing to Toriel, or his memory of her, because he had to break that promise.

This post is a mess but... I need to rotate them. And also I always get a little mad when people assume that Sans doesn't make that promise when Toriel dies, like in No-Mercy. Because the whole deal about the promise is that it happens before the game, it is not something anyone, not even Flowey, can affect, and it is a reflection of the closeness the two have, so placing this as something Toriel does only if she deems the human a good person, and something Sans only "keeps" when Frisk is pacifist just takes away that weight.