Blue had a secret.
Well, it would be a lie to say he had only one. Like many, he had little things he didn't want others to know but weren't too important either. Like how he actually didn't like cooking. But if it was the only way to convince Papy to eat healthy food instead of those sugary, cavity inducing pastries from Muffet's, then he would gladly lie to his brother forever and ever.
Or like it was more or less a lie that his biggest dream was to join the Royal Guard. Sure he enjoyed training with Alphys and was doing his best to be accepted but.. Truth to be told, he wouldn't even consider it if it didn't pay well enough for Blue to be sure there would always be a roof over Papy's head and food on his plate.
And those little lies weren't for his own sake, Blue reminded himself whenever he prepared the ingredients for his tacos or when he was about to patrol Snowdin's forest. No, they were all for Papy, to make sure he lived a comfortable life.
No, Blue's real secret was much more... personal.
Blue was in love.
But he couldn't tell anyone, not even Papy.
It all began one year ago when the mysterious machine that was lying dormant in the basement of Blue and Papy's house activated itself and opened a portal to other universes. After several weeks of testing by the Royal Scientist, Undyne declared it safe to cross the portal, and Blue and Papy began traveling through dimensions to explore all the alternate universes that were linked together through the machine.
In the following months, Blue discovered many worlds, familiar but so different from his own. Every universe was similar, in some way: same monsters, same places, same history, except for little... tweaks here and there. In the Undertale world (as labeled by the machine) for example, every monster had swapped personalities. The Sans was lazy and liked to tell jokes, just like Papy, when the Papyrus was very great and hard-working, almost as much as Blue himself!
And yet, it was Blue's universe that got labeled 'Underswap'...
After meeting so many different Sanses and Papyruses, it became obvious they needed surnames so as a general rule, the Papyruses took the name of their respective universes while Sanses got names after the color of their magic. That was how Sans got his nickname, 'Blue', which he really liked because well, blue was his favorite color! There was no way he was going to call his brother 'Swap' though. Papy was Papy and that was it.
The nicknames got particularly useful when PAPYRUS began to throw parties in his home in Undertale. He would invite Sanses and Papyruses from multiple universes so everyone could get to know their alternates and make new friends.
That was actually at one of those parties that Blue met... him.
Papy was sick that day, and Blue had to come to PAPYRUS' party by himself. Not that he couldn't have a great time even without Papy, but that day most of the guests were from Fell types universes that Blue didn't really know. It soon became very obvious to Blue that he didn't fit into any of the conversations happening between the many Papyruses and Sanses present, and PAPYRUS was too busy making sure the food was as great as he was to have Blue bother him all night. He couldn't even stay with Sans, who had fallen asleep after only two cups of spider cider.
So Blue ended up standing in a corner of the living room, sipping a cup of juice as he waited for the time to pass, too polite to just leave the party like that but not bold enough to randomly gatecrash into one of the conversations.
"hey," a deep voice suddenly drew Blue's attention. "how's ur fever?"
Next to him was standing a Sans that Blue had never met before. He was wearing a sleeveless purple jacket, had a drink in his hand, and a casual smile on his face that told Blue he was waiting for a reply.
