Sans was worried that Papyrus might be upset to learn that his big brother wasn't really related to him. He had just shrugged - they were both skeletons and there weren't many skeletons around so they were probably related in some way after all, and besides, he could pretend he was the Crown Prince and his brother was the adopted Prince. Most of all, he just found the story fascinating, like a fairy tale.
"Back then, I wasn't called Sans," he told his brother, both of them sprawled on their battered but comfy old sofa, "I was called Dem."
"Like... half, instead of none?"
Sans nodded, sad longing in his one flaming blue eye, "There was a girl called Sara. She was the other half of my being."
"I never figured you for such a romantic, bro! I still say you should put in a good word for me with Undyne."
"Except that's not how you impress girls and anyway Undyne isn't interested in goofballs," or men, he mentally added, before sighing, "And I'm the worst person to ask. I lost her. Everyone, and our world. I wasn't given the option to save them. Do you understand? The options in the dialogue box..." he sighed again and shook his head, "No, you wouldn't understand what the hell I'm talking about. It's advanced higher-dimensional metaphysics. I'm the dummy for thinking that anyone else understands things like that," Except Gaster, but I lost him, too, on my own selfish quest to find Him and punish Him.
I've found out where he goes, though, I can track Him. He has to come out, you can't live in a debug room, and anyway he ate Toriel's phone.