The theme of the game (And nature of the Human Soul) is to "Right wrongs of the Past and Defy and Change Fate". The True Lab posts talk about this explicitly. Asriel himself says in their battle that Frisk "Never Gives Up, because they want a Happy Ending, Because they Love their friends". The final part of the battle has Frisk realize that "There's still one more person that needs to be saved" and the rest of the Battle is about "Saving" Asriel. Frisk also previously escaped from the Underground in the Neutral playthrough but willingly reset in order to find a way to get a "Better Ending" "Save Asgore" meaning they were willing to give up their life above ground in order to save the life of someone seemingly unsavable. Asriel (as Flowey) even states in repeated Neutral Endings that "Frisk is trying to become friends with him" showing that Frisk feels sorry for Flowey even when not knowing the kind of person he truly is and that they want to help them somehow.





Throughout the final battle Frisk reaches out to their friends and Proves to them that their fears and worries are not true. Frisk is actively defying their fears and expectations and promising them that everything they fear is unfounded and that Frisk will make it all right. Alphys is scared that everyone hates her, but Frisk proves to her that's not true. Toriel is scared that Frisk will die if she doesn't "Fight them to save them" but Frisk proves that's not true. Asgore is scared that "He has no choice but to kill Frisk" but Frisk proves that not true. Undyne thinks that all Humans are bad and that she must destroy all of them, but Frisk proves that's not true. Papyrus is scared that he has no friends but Frisk proves that's not true.





The Last one other than Asriel is Sans who has "Given up because he thinks he'll never see Gaster and his colleagues again "You'll Never see them again". This is one that isn't shown getting fulfilled but it is implied because of Sans giving Frisk the key to his basement which lets Frisk find out about Gaster and his colleagues". Frisk isn't the type to give up and when they find out someone's in trouble they don't stop trying to Save them until they succeed. Even death doesn't stop them.





All of this is proof that Asriel, Gaster, and the colleagues will be Saved as well.

