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Reminiscence is an AU covering the origins of Sans and Papyrus. It is set after the True pacifist ending and is told by Sans to Frisk (and accidentally to Papyrus) while sat at Grillby's.
Child Sans cheats and steals in order to make ends met, using his teleportation magic to get out of trouble when caught. His HP and DEF is far higher than 1. He and Papyrus live in a run-down shack in New Home.
Child Papyrus is as small and adorable as would be expected. He is not seen much in the series to date. Sans ends up looking after him alone at a very young age.
Sans and Papyrus's mother. Powerful, intelligent and having a love of puns, she raised Sans and Papyrus alone until the war took her life. Sans promised her that he would look after Papyrus.
W. D. Gaster
The royal scientist is cold and easily annoyed. He clearly has something to hide. He hires Sans after Asgore insists and Sans fixes a machine.