Trust me, when Jasper shows up in the show again, it'll go something like what's at the end.
Just you wait.
Flowey the Flower sat in the underground, looking up at a shaft of light from above that had been created from one of many recent earthquakes. Lord knows what an earthquake in the ground would look like, but no one cares about the logistics of tectonic plate shifts from the perspectives of said plates.
Suddenly, someone fell from the surface and landed loudly next to Flowey. It was Jasper.
"Howdy," said Flowey. "I'm Flowey. Flowey the flower."
Jasper said nothing, because imploding from a gem fusion back into your original form apparently knocks you unconscious for a while. Because exposition.
"You're new to the underground, aren't you?" Flowey asked, even though he knew the gem wouldn't respond.
Suddenly, Jasper woke up, because plot. "ARGH!" she shouted loudly. Suddenly, the shaft of light closed up, because convenience.
"Aw, man," said Flowey, "now you'll have to travel through the whole underground so I can steal your soul—GAK!" Jasper had rolled onto her back in frustration, crushing the unnoticed flower to death.
"Hello?" a voice called. Jasper looked up to see Toriel, who had heard the commotion and come to investigate. "Oh, greetings," she said when she saw the gem. "Are you a human?"
Jasper's nose lit up. This creature was tall, wore a long dress looking thing, and seemed very maternal.
"ROSE QUARTZ!" Jasper screamed as her helmet formed on her head. "YOU THOUGHT YOU COULD HIDE YOUR GEM FROM ME BY COVERING IT? YOU THOUGHT WRONG!"
Jasper threw herself at Toriel. Toriel screamed and threw a few fireballs at Jasper before running away.
"DANG IT!" Jasper fell on the floor and pounded her fists in defeat. "FOILED AGAIN!" Jasper suddenly blinked at a scorch mark on the dirt from a stray fireball. Jasper swam across the floor for further examination.
It was a dark mark of ashes mixed in the dirt. She looked at it. She smelled it. She ate it.
"Hmm," she said, no longer shouting as much. For now. "Elemental powers… rOsE qUaRtZ is even more powerful than I thought if she has learned these powers. I MUST FIND HER!" Jasper back flipped onto her feet and ran after Toriel.
Being a gem without a real physical appearance, Jasper was unaffected by the puzzles or inhabitants of the Ruins. In a minute, she was at Toriel's house.
"ROSE QUARTZ!" Jasper yelled. She did a Sonic the Hedgehog and leapt at Toriel. Toriel screamed as Jasper ran her through. She turned into dust.
"DANG IT!" Jasper said as she scooped the dust into a nearby bowl before eating it. "FOILED AGAIN!"
"A decoy…" she muttered as she shoveled dust into her mouth. "RoSe QuArTz is trying to hide from me… which must mean that she's here! I MUST FIND HER!"
Jasper threw the bowl behind her and ran to the entrance to Snowdin. She barreled past the conveniently shaped lamp, past all the puzzles, and into the town. Monsters screamed as she stormed through, and anyone who didn't move in time was crushed under foot.
"OMG!" A voice in front of her shouted suddenly. Jasper screeched to a halt. Papyrus stood in her way, bouncing excitedly on his heels. "Sans, is that a human?" he asked. Sans was nowhere to be seen. "Sans?"
This creature had a cracking voice and an optimistic energy. Jasper was reminded of when she became half of Malachite.
"ROSE QUARTZ!" she screamed and launched herself at Papyrus, who screamed as he crumbled to dust.
"DANG IT!" Jasper roared. "FOILED AGAIN!" She quickly mixed the dust with snow into a snow cone and ate it. "rOSE qUARTZ must be close by. I MUST FIND HER!"
Jasper stomped through Waterfall, squashing Echo flowers and Temmies as she went. "Gems are… not cute!" A Temmie wailed as it fell off a cliff. Suddenly, Jasper saw Undyne standing up ahead of her.
"Halt!" said Undyne. "Are you the creature that's killing everyone? I'm going to defeat you! I'll protect the rest of the Underground and my friends!"
This creature had strong friendships and the intent to protect a whole world from its fate.
"ROSE QUARTZ!" Jasper yelled and crashed into Undyne.
"Noooo!" said Undyne as she crumbled to dust in the wind. Suddenly, her dust melted into a puddle and she became Undyne the Undying. "Ha! Now I'll defeat you!" Jasper stuck a crazy straw into Undyne the Undying. "Noooo…" Undyne the Undying said as Jasper drank her.
"DANG IT!" Jasper tossed her straw like a javelin and killed Napstablook. "FOILED AGAIN!" Jasper looked at the shiny castle in the distance. "R0s3 Qu4RtS must be closer still! I MUST FIND HER!"
Jasper ran through Hotland, jumping from monster to monster and stomping on them like in Mario Bros. Suddenly, Mettaton EX appeared, smelling like Mettaton.
"Darling, your nose," he said, kicking a boot in the air. "It's gorgeous!"
This creature had pink.
"ROESSU KWAHTZU!" Jasper said all anime and did her attack in anime fashion.
"Uwah!" Mettaton said all anime like and blew up.
"DANG IT!" Jasper said as she bit into a piece of Mettaton and chewed it angrily. "FOILED AGAIN!" She looked forward and saw that a wall of the mountain was making itself apparent through the mist. "There's not much room left to search. Fl0w3r R0ck must be here somewhere. I MUST FIND HER!"
Jasper did another Sonic and rolled forward at the speed of sound. She rolled the dice and went forward as many spaces as directed, killing each monster that was unfortunate enough to have been volun-told to be a space. Suddenly, Jasper was in the Judgement Hall, with Sans at the far end.
"Heya," said Sans.
This creature was humanoid.
"You've been busy, huh? Well, here's a question for—"
"ROSS QUARTERS!" Jasper shrieked and blasted Sans to pieces like a bowling pin.
"No fair!" Sans whined as he turned to dust. "You didn't let me say my cool speech and stuff!"
"Where is ROSE QUARTS?" Japers asked.
"Dude, Robert's Son was never here!" Snans replied. "Go back to the surface, you murderer!" He turned to dust.
"DANG IT!" Jasper said. "FOILED AGAIN!" She looked up at the ceiling. "Always a step ahead of me, eh, Rope Corners? This was just a cheap trick to distract me! Well, there's nowhere left to run, now!"
Jasper flew through the ceiling and appeared to the surface. Because Jasper was a sentient rock, the specifics of her supposed soul were too complicated to be bothered with in relation to the barrier and she went through it with ease.
Meanwhile, on the surface, the Crystal Gems were doing a thing. Suddenly, Jasper came up through the ground and interrupted the thing.
"Oh, snap!" Steven said. "Jasper's back!"
"I FINALLY FOUND YOU, ROSE QUARTZ!" Jasper yelled evilly.
And that's how Jasper killed/ate everyone in the Underground, thus preventing the complications that a canon crossover like this would create to the plot of both Steven Universe and Undertale.
The End