Disclaimer as always: I don't own the game. I only made the story.
"Hi, I'm Flowey! Flowey the...Um..." The flower trailed off and just kind of stared. This...wasn't what he had been expecting. The cuteness was unbearable. He couldn't stop staring at it. It was filling his eyes, petals, nonexistant soul. He. Could. Not. Resist. "Oh mah gaaaaawd!" he screamed. Then he suddenly became adorable.
Frisk picked her way through the Ruins. She bumped into a weeping, flying creature. "Butterfwy!" she squealed. The Whimsum was instantly consoled.
"Toriel! Toriel! I have to...have to warn you!" Flowey panted. Toriel looked at him quizzically, but it was too late. Frisk came through the door, and everything became sparkly and pink. Chara put down the weapons and the chocolate and started going crazy like a noob on Minecraft who has just fount nineteen diamond blocks and a nether star.
"Do you wanna have a bad time?" Frisk didn't answer right away. Slowly, she shook her head. It was too much for Sans. His eye started flickering from blue to yellow to black and then it suddenly became kawaii.
Papyrus hung out with Frisk pretty much like normal.
"So you're the human whose been...been..." Undyne couldn't move her spear another inch. It was too cute. The human was just too cute. The fight ended right about there.
Muffet looked up from her stand. A tiny girl was waiting at the front of the line, trying to count out 9999 gold. Muffet looked at the four coins in her hands. "Is this enuwf?" Frisk asked. Muffet's five eyes became kawaii and she sent Frisk along with a cupcake.
Alphys heard the door to the lab open. She thought she had been prepared after seeing the video feed from the cameras, but no...This was more chibi, more sparkly-eyed than the cutest anime that she had ever seen. Mettaton came in. "Oooh! Computer!" Frisk squeaked. All the amalgamations melted a second time.
Mettaton transformed into Mettaton EX. "Oooh, pwetty computer!" exclaimed Frisk. The ratings went up by three billion.
Asgore was at the Barrier. He was at teast ten times the size of Frisk. Frisk looked at Asgore. Her eyes were filled with tears. Her tiny body trembled. Finally, in a small, frightened voice, she managed to squeak out, "Mr. Goat, are you gowing to eat my sweater?" Asgore instantly melted into a rainbow.
Moral: Too much determination turns you into an amalgamation. Too much cuteness turns you into cuteness. I am depressed, and had to write this.