Notice: This contains spoilers of Demon Slayer and Undertale! Read at your own risk!
Also, Frisk is a boy and Chara is a girl. Frisk is 15 and Chara is 14 currently. Don't mind the flashback parts.
Two years had passed ever since Chara was transformed into a demon, and Asriel a talking flower. Frisk could still remember clearly, on that very day, when he had found Toriel and Asgore dead...
Two Years Before...
His family was dead...although both of Chara and Asriel were still warm...he had to at least find help for them...quick...
I have to find help...but carrying both of them...such a burden...I can't...but I have to...
Frisk gritted his teeth and continued on in grim determination, but he could feel himself slowing down with every step he took.
Suddenly...he heard a growling noise...coming from undoubtedly Chara...
Chara tumbled off of Frisk's back, still wildly thrashing.
Asriel slid off of Frisk, but he had no choice but to rush over to Chara.
"Chara! Chara, are you alright!?"
Chara leapt up and charged towards Frisk. She now had red claws as fingernails, black scleras, and a black substance dripping from her eyes.
She madly hit and scratched at him. She was incredibly strong now. It bewildered Frisk to see how Chara's strength had increased. Until he had remembered what Old Man Papyrus had told him...
The Night Before...
Frisk was traveling up Mt. Ebott after delivering Toriel's homemade Butterscotch Cinnamon Pies to Snowdin Village below the mountain. He was extremely tired, but content and relieved. They would get ready for the SOUL Festival tomorrow.
Along the way, he came across Old Man Papyrus's cabin.
"Hey, friend, would you mind sleeping here for the night? It's gonna become extremely cold and especially dangerous out here at nighttime."
Frisk hesitated. He knew Old Man Papyrus very well; He had been good friends with his family, but he really didn't want to spend the night.
"Uh, no thanks!"
"Friend, it's gonna get dangerous out here, so I suggest you really come inside."
Frisk thought for a moment.
"Alright, sure then!"
Papyrus smiled and welcomed him inside.
"It shouldn't be wise for you to prance around in these woods on your own at night, especially for someone so young like you."
"Why? What makes the woods so dangerous?"
Old Man Papyrus stirred a stew over the fire. "Well, there are man-eating demons in these woods. Demons that will tear apart any human they find. 'Course, they don't ever go after monsterkind like me, but then again, not a single soul in this world knows about monsterkind but you. Sure, I appear to be human, but no one knows I am truly a monster."
Papyrus set down the bowl of stew on a small table, along with two bowls and a big plate filled with his famous spaghetti.
"Enough talking. Let's eat."
Later At Night...
Frisk lay on his side on the spare mattress Papyrus had provided for him, lost in thought.
Demons? What are they? What do they look like? Why do they devour every human they find? How did they come to be?
How many did they already kill...? Why?
What will become of us if we ever encounter one? My family, my friends?
Frisk fretted himself with all of these questions until finally, he fell fast asleep.
That was two years before. Two years before Frisk went to the Final Selection.
Present Day: Final Selection
Frisk's eyes opened to find Master Sans standing over him.
"Frisk, it is time."
Frisk obediently got up, knowing he had to head to the Final Selection.
"Frisk, look at me."
Frisk turned around and looked at Master Sans.
"Make it back alive. Your sister and I will be waiting for you.
Frisk thought longingly of Chara, who had slept for two years, trying to regain her strength without killing and eating any humans. Flowery/Asriel quickly looked at him from the little table, and looked away. Then Frisk nodded.
He quickly got ready. But before he left Master Sans's cabin, he yelled "Say hello to Bravery And Patience for me! They helped a lot during my training!"
Frisk was standing before a spider woman, with hundreds of other people coming for the Final Selection.
"Greetings, everyone. I am Muffet." The spider woman spoke in a thin, silky voice.
"Welcome to the Final Selection."
"These spider webs are designed to keep the demons from coming out, you see. In order to pass the Final Selection, you have to go in there, and survive for seven days. Obviously, there are demons in there, and they are hungry."
Muffet continued. "We have made arrangements for another Final Selection, but with Wisteria flowers. Meaning that there is another Final Selection near Mt. Sagiri. There is a portal in there that will teleport you to that Final Selection. The specific instructions are for you to stay in this Selection for 4 days, and then find the portal, and move on."
Frisk calculated it all in his head, then took his first step into the Selection area.
After Slaying Several Demons...
Frisk was exhausted but determined. All his hard training had paid off.
But before we go on, I would like to explain something about Frisk's pillar.
There are unknown pillars that very little people know about. Only the monsterkind know about them. They are the pillars of Determination, Kindness, Justice, Integrity, Bravery, Kindness, Perserverance, Patience, LOVE, Fear, and HATE.
HATE is the most dangerous pillar among all of them. The pillar seeks to control and eliminate all. HATE pillars can be indentified easily in a crowd, with their black skin patches and blank, soulless stares. It is one of the most powerful pillars, yet so dangerous. And yet, very little people know about it. That's what makes it even worse.
Frisk is the Determination pillar. But during his birth, something has gone horribly, horribly wrong, and HATE, LOVE, and Determination all clashed on Frisk. Some of it was made into Chara, but only a little. Chara turned out to be a caring, nice girl.
LOVE is very different from Love. Love is beauty. LOVE, however, is an acronym. An acronym for Level Of ViolencE. Someone's capacity to hurt and kill.
Four Days Later...
Frisk was now standing before the portal that would take him to the Final Selection with Wisteria flowers.
He went through.
There was a lurch, as if the world was turning upside-down, and Frisk landed in a gloomy looking forest. He may have survived the other Selection, but he might not survive this one, so he has to be extra careful.
Gripping his katana, he heard a commotion that was coming from behind the trees on his left.
He zoomed behind a tree.
He saw a boy about 15 with burgundy hair slaying a demon. In fact, 3 demons in a row.
Okay, I didn't have to go and investigate, I could've just minded my own business-
The boy zoomed over behind him, and sliced off a demon's head that lurked ominously behind Frisk.
There were the two of them, slicing and dicing any demon that came their way.
They stared at each other.
"I guess I'll see you later."
Tanjiro zoomed off.
Frisk continued killing every demon that attacked him, slicing their heads clean off of their shoulders.
Suddenly, a horrible, putrid smell filled the air.
"Agh! What the heck!?" Frisk covered his nose.
The smell seemed to be miles off. But Frisk's curiosity got the better of him, so he zoomed off.
What was awaiting his sight was Tanjiro, lying out cold on the ground, and a horrible, ugly sliming demon with hundreds of arms. One move and it would all be over.
"Hee hee hee! There goes another one of Urokodoki's brats!" the demon cackled. "I'm going to crush his head and swallow him whole! Ha ha ha!"
I could just hide here and avoid him. Frisk thought, his heart pounding out of his chest. I'm hidden perfectly. I don't need to put myself in danger.
But...he saved me.
Suddenly, he heard Master Sans's voice echoing in his head.
"You are a Demon Slayer. You help people. You don't stand by and let the demon win."
As though he was planning it all along, Frisk did Total Concentration Breathing. A burst of red Determination lit up his katana, and he leapt out, right at the demon.
In lightning fast slashes, he cut up the demon into tiny pieces, due to fueled Determination and LOVE.
Frisk landed neatly in the center of the clearing.
Tanjiro had woken up at this point and was shocked. His face has gone deathly white.
The small patches of HATE that had formed at Frisks's elbows and knees faded away, and although Frisk wasn't aware of it, when he did Total Concentration Breathing, his sclera had turned black and his eyes, which were red, turned ominously darker, and now were going back to normal.
His sword also dimmed.
H-How!? thought Tanjiro. There isn't a pillar like that!
It occurred to Tanjiro that this kid was way more than he appeared to be.
To Be Continued...