Frisk wandered into Grillby's, looking around for Sans. They had found a really pretty flower and wanted to show him, but the second Frisk saw Grillby, they rushed over to him instead. Climbing up onto the stool, they held out the flower to Grillby.
"Look!" They said, bouncing up and down. Grillby put the glass he was was cleaning away and leaned over the counter to look at the flower, even taking it in his hand when Frisk insisted he have it. Despite him being literally made of fire, the flower didn't burn as he looked more closely at it, and to be honest no one knew why.
"I love it. Thanks, Frisk." Grillby said, voice deep but clear. His flames grew slightly brighter in joy, and he reached over to ruffle Frisk's hair. The door burst open just then though, and all eyes were on the ever fabulous Mettaton standing in the doorway.
"Oh Frisk darling! Your favorite uncle Mettaton's here!" Mettaton shouted, walking over towards Frisk. Grillby's flames flickered in annoyance, but he simply went back to cleaning the rest of the cup. "Oh my darling, what ever are you doing in such a... Er, cozy, restaurant?" Frisk didn't really notice the question though, instead just jumping off the stool and hugging Mettaton's leg. Grillby's flames flickered a little at Mettaton's comment, but he kept his voice calm as always.
"Do you need anything Mettaton?"
"Oh, hello Grillby. No, I'm just here to pick up Frisk to be the special taste tesing guest on yours truly's fabulous cooking show!" Mettaton struck a rather overdramatic pose, but finished it off by tightly hugging Frisk, who of course did their best to hug as much of Mettaton as possible back.
"The food here would be better," Grillby commented evenly, not taking his eyes off the cup. Mettaton froze, pursing his lips and all but full on glaring at Grillby.
"I doubt it. Besides, I'm sure Frisk would rather spend time with their favorite uncle Mettaton, isn't that right dear?" Mettaton asked. Frisk was clearly confused, but before Grillby could say anything else Mettaton was eagerly dragging a baffled but excited Frisk away.
The next day, Grillby woke up especially early and made his way over to Sans' and Papyrus' house, which was also where Frisk lived. There weren't many people out that early, but when he knocked on the door he hardly had to wait any time at all before the door swung open.
At the door was Frisk, still looking sleepy but instantly perking up when they saw Grillby. He could smell something... Not very appetizing, coming from inside, but managed to overlook it to stay cheerful.
"Frisk, I was wondering if you wanted to join me for my lunch break today...?" He asked quietly. He didn't consider himself to be the best with children, but he had been honest and straightforward, right? So surely there wouldn't be a problem. Unless Mettaton actually was Frisk's favorite uncle. Grillby winced at the thought, but Frisk just nodded happily.
"M'kay!" Frisk rushed back into the house, and Grillby could hear them talking to Sans and Papyrus. Frisk hadn't closed the door, so before he left he gave a small wave to Sans and Papyrus before gently shutting the door and walking back to work.
Everything was relatively quiet in the morning, but the later it got the more customers came in, and not too long before his lunch break there was a huge rush of customers. He continued working briskly though, determined to not have to delay his lunch break. It wasn't an especially odd thing for him to do, but he most definitely couldn't expect Frisk to wait for him.
It was tiring, but by the time noon rolled around all the customers were taken care of and eating at their tables. Sighing and turning around, Grillby jumped a little when he saw Frisk sitting at the counter, tracing lines on it with their finger.
"F-Frisk?! How long have you been waiting there?! I'm so so sorry, I didn't mean to make you wait, I, umm... I'm sorry! Please forgive me!" Grillby was totally caught up in apologizing, rambling far more than he usually would, but who could blame him? He honestly thought Frisk might be annoyed at him for making them wait, although he'd never ever do anything to upset them on purpose.
Frisk only took a moment though before shaking their head and smiling at Grillby. He was a little uncertain at first, but eventually gave in, sighing in relief and quickly leaving to grab the lunch he had made for himself and Frisk not too long ago.
Just as he walked into the back though, he heard the front doors slam open, and an unfourtunately familiar voice shot out through the whole building. Quickly grabbing the lunches and walking back out, Grillby managed to slide a seat over on the opposite side of the counter as Frisk before Mettaton could get to them. Of course, it didn't really change much, but it made Grillby feel more like he was already doing something with Frisk so Mettaton might just leave.
Of course, that didn't actually happen. Grillby couldn't tell if he did it on purpose or just really didn't see him, but either way Mettaton walked up and gave Frisk a hug without so much as a wave in Grillby's direction.
"Frisk darling, your favorite uncle Mettaton's here to take you out to luuunnnch!" Grillby winced at the robot's overly loud tone, but mostly at his use of "favorite uncle". Surely Frisk hadn't actually told him that? I mean, Grillby didn't think especially highly of himself, but in his opinion Mettaton was loud, kind of rude, and entirely overdramatic. But that was just his opinion. He supposed Mettaton did have some redeeming qualities as well, but that was probably why he felt so threatened by him.
"...She's having lunch with me. It's quieter and more relaxed that way." Grillby didn't even mean to let the second part slip out, but Mettaton was clearly offended.
"And what do you mean by that?" He asked loudly, glaring a Grillby. Grillby himself was prepared to give a very uncharacteristicly rude reply, but remembering Frisk was in the room, he didn't let his agitation show in his response. Or at least not much.
"Look, all I'm saying is that you seem to just assume you're Frisk's favorite. I for one have never heard them say that, have you?" Based on Mettaton's flustered response, Grillby was guessing not.
"Well...! How about we ask Frisk then?!" Mettaton was even louder than before, and by then all eyes were on the growing argument. Both men instantly looked towards Frisk, who seemed incredibly confused.
"So darling, which uncle do you like best?" At least Mettaton had lowered his voice some, making sure that his annoyance at Grillby didn't carry over to Frisk. Grillby didn't say anything, but was practically holding his breath all the same. He knew it was silly, but still, was he really less likable than someone as loud and dramatic as Mettaton? Wait wait, no, maybe Frisk liked that about him. Actually, he reminded himself, he shouldn't be judging people like that at all. And yet he still remained nervous.
Frisk pressed a finger to their lips, clearly deep in thought. Mettaton's confidence slipped for a second to reveal an equal amount of nervousness. The whole room went practically silent as everyone waited to see the outcome.
"Uncle Sans!"
"...What?" Both Grillby and Mettaton spoke at the same time, wearing matching shocked expressions. Frisk didn't really get the question though, and instead just continued smilng, looking from Grillby to Mettaton and back.
Mettaton was the first to recover though, laughing a little.
"Hahaha, oh my, I guess I should have seen that one coming, hm?" He ruffled Frisk's hair, although it was clear that the idea hadn't even occured to him once. Grillby slowly resumed moving, and decided to get warm drinks for both Frisk and Mettaton.
"Yeah... I mean, you do live with Sans. He's pretty cool then, huh?" Of course, Grillby already knew that Sans was more than pretty cool in the eyes of, well, basically any child ever, but he needed to say something at least.
"Although..." Frisk murmured before taking a sip of their drink. "There's a second," But with that Frisk ran off, giggling like crazy.