(This is my Third Sans x Older!Frisk book, so it'll probably be better than the others. Undertale by Toby Fox. Also in this the monsters don't go above ground, they still live underground.) After Frisk saved the underground, she didn't act the same. She wasn't the same happy and cheerful Frisk. She acted depressed, lonely even. She didn't seem to enjoy the monsters' company anymore. Sans has fallen for her, but he knows that he has to do the only thing to make her happy.