It's been eight years since the barrier went down, Sans thought absently to himself, he reflected back on the years, watching Frisk grow from ten to eighteen had amazed everyone as monsters didn't age the same way humans did. Toriel excitedly marked every time Frisk grew noticeably on a door frame as she had seen on a variety of human TV shows, Frisk happily complied every time even as she became much older then the children from the shows, even letting Toriel continue to check for a year after she had stopped growing. She grew to almost as tall as him.
She attended classes at school almost religiously all the way until she graduated, even when she outgrew the grade school Toriel taught at and started into high school, she had taken up a few hobbies, writing and drawing, she had a blog and chatted with a lot of people from far away on the internet. But one of her hobbies eluded Sans; she took up playing the piano at age twelve, but wouldn't let Sans take her to her lessons of be at her house while she was practicing, it was almost like she was preparing something he wasn't allowed to see, it confused him but he respected her decision.
"Sans!" Papyrus yelled from the other room, "we're supposed to be having dinner with Frisk and Toriel in fifteen minutes, are you ready?"
"yeah bro, don't I look ready?" Sans responded not moving from the couch.
Papyrus entered the room and scrutinized him from the doorway, wearing a bright yellow graphic tee that read 'cool dude' in big green letters and a pair of bright blue shorts he picked up when Frisk last took them to the department store nearby. "You look like you're wearing what you always wear." He judged.
"then I guess you can say I'm good to goat!" Sans retorted, chuckling to himself.
Papyrus glared, gritting his teeth. "that was a stretch Sans." He replied dismissively.
When they arrived they we're greeted by Frisk opening the door, signing excitedly about having lasagne for dinner.
"I'm always happy to try a new type of spaghetti." Papyrus exclaimed, Frisk nodded happily, she had given up on explaining that spaghetti wasn't the origin of all pasta dishes long ago.
Papyrus left to help Toriel in the kitchen and Frisk motioned for Sans to sit on the couch with her, they joked around and watched TV until Toriel called them to the table.
Dinner was the same affair as every week, joking and banter, there's normally more people but everyone else was busy this week, Undyne was helping Alphys with some study at the collage she worked at, and Mettaton was on tour till the end of the month.
Toriel spoke up, "You know boys, Frisk has a piano recital coming up. You should come."
Frisk coughed as she choked on a bite of lasagne, everyone sat silent for a moment while she took a drink of milk before signing frantically at Toriel.
"Why not?" Frisk signed some more, Sans caught 'you know why.' "No sweetie I really don't, I'm sure they would love to go see you, you're the best in your class."
She signed some more, Sans recognized something like 'not good enough', Papyrus spoke up "You can't be better then the best Frisk, I should know, as I the Great Papyrus have heard your music and also recognize you as the best."
She huffed, Sans spoke now "You know kid I'm feeling pretty left out, why don't you want me to hear you play?" He mentally kicked himself for sounding so hurt.
Her expression sank as she fought back tears, 'I'm sorry.' She signed, before standing and going to her room, leaving her food half-eaten to get cold.