Classic: You were not in the mood to cook, and you just got paid, so you decided to go to Grillby's. (On the surface. we're going to say he has a surface location.) You looked around, and the only seats tht weren't taken were at the counter. You took a seat at the counter and awkwardly watched Grillby clean glasses  like some creep. "Hey. What would your name be, kid? Mines Sans. Sans the skeleton." A voice said. After  moment you turned to look at the owner of the voice, realising they were talking to YOU. You introduced yourself and you both chatted nd exchanged terrible puns and bad jokes. Before you left, the both of you had exchanged numbers.
Swap (blueberry) : You were meeting up with Papyrus at Muffet's place, but what you didn't know was that he had brought his brother with him. You didn't know he even HAD a brother. But he introduced you to him and you guys were instant friends. You exchanged numbers and left, staying in contact and hanging out a lot, making tacos, training together, he even let you sow up his scarf when it ripped.
Fell: You sat down on a barstool at the surface location of Grillby's. You turned  to your left and saw a skeleton with a red, gold, and black outfit, a little crack on right side of his skull. "What the fuck ya starin' at?!" He growled. You shrugged. "I dunno, you, I guess... "
His demeanor  hanged, something that offset, but also... Enticed you. "Uh, the names Y/N." you mumbled, barely loud enough for him to hear. "Mine's Sans" He said, his voice scratchy. You two chatted for  a while, and you started showing up to the fire monster's resteraunt, and each time he was there.