So this idea came to me one day and I'm surprised nobody else thought of this. Only one piece of art. The closest I found was a fic called "Kids like you shouldn't be burning in hell" by Piggyslayer1235 and that was with Ruby as Sans only. I don't own Undertale or RWBY, if I did this would be canonical to RWBY and Undertale.
Also major spoilers for all 3 endings of Undertale and some spoilers for RWBY. You've been warned.
It had been 60 resets now, Frisk has seen all the endings and where they lead. No matter of daunting or horrifying that Genocide Route might be, it was one of the final ways they knew in order to learn about the world of the Underground. It also final gave them closure on that dusty route. So after taking every option possible. Frisk became curious about the Skeletal Brothers Sans and Papyrus. Not to mention that W.D. Gaster fellow they heard about
Eventually on this Pacifist Route, they would try to learn more about them. No matter what they would find the true history of these two. Even if they had to reset 1,000 times.
So once they could reach and greet Asgore, Frisk turned around after being judged by Sans, earning a key to his room in the process, Then went back to Snowdin town to find Papyrus and Undyne. Ready to go get Undyne's letter.
"Hello human, I, the Great Papyrus, wonder what are you doing here.
"Do you know where you and Sans came from?" Frisk asked using sign language.
Papyrus then looked confused before saying "Well isn't it obvious, we came from Skeleton monsters."
Frisk raised an eyebrow in confusion, but since Asriel was a thing. They just went with it.
Undyne shot him a confused look. "Paps, I don't know if that's how skeletons work. Maybe you and Sans were made in a lab and Sans just lied to you?" she suggested.
"No, that sounds far fetched. Sans hasn't lied to me. That I can confirm." Papyrus said sweating. Frisk could tell he was getting uncomfortable about this and tried to change the subject.
"Undyne didn't you want to hand Frisk a letter?"
"Oh crap right, thanks Papyrus." Undyne thanked.
Undyne then handed Frisk a letter that was meant for a certain Dr. Alphys.
Frisk nodded as they ran off. Undyne then left as well, going out to buy some new clothes for the date.
Papyrus then smiled as he and Sans were now alone. Fianlly he could cut the overly cheerful and nearsighted facade. He then went to his room and looked in his closet.
He then pulled out a box that contained a few things.
A red hood with a black and red outfit for a 15 year old girl.
A red and black scythe weapon.
And finally, a photo of a young human girl with red and black hair wearing the outfit with notably silver colored eyes, an older girl with long white hair that was tied into a ponytail and silver colored outfit and faded blue eyes, a similarly aged girl with black hair with a black bow with a black and white outfit and yellow cat-like eyes, and finally another older girl with long messy blonde hair with a scarf and brown outfit and lilac colored eyes.
He smiled at the photo as he sheds a tear.
"It's been only 2 years down here for us, but I know our deaths haven't technically happened up there yet and will in a few weeks. When me and Yang get out. We'll come find you Weiss and Blake and we'll save ourselves from death."
"Ruby?" Papyrus hears as he turns to see his brother Sans... no, her SISTER Yang. She had already turned off her disguise and was now in her taller, thinner, bustier, beautiful human form as she know looked the blonde girl in the photo. She still wore the blue hoodie, black shirt black pants and pink slippers, but her hoodie now had a fully black emblem of a flaming heart adjacent to her real heart's side.
Ruby then smirks and her left eye burst into a cresent red rose-shaped flame as her form shifted. She turned into a shorter, younger, human girl. She now became the young girl with the silver eyes. She still wore her battle body, but the royal emblem became a flowing red rose shaped symbol and her scarf was longer, but considering that it wasn't a planned part of the outfit until a month ago when Yang gave it to her for her birthday, Ruby then smiled with pride as she looked at her older sis and hugged her.
"I wish we could go out and be ourselves for once." Ruby said sadly, knowing that isn't gonna happen not with 6/7 SOULS gotten. It was only because of the special connection they had to a certain woman that was lucky to be here that they could stay hidden. So far only that woman and Alphys knew their secret. Alphys swore that even if they died, she would take the fact she knew they were human to her grave unless the time was right and they agreed to tell everyone themselves.
"Someday sis. Someday." Yang said as she and Ruby saw Zwei, cover in snow making him appear a pure white color. The two giggled at the adorable corgi and began to play with Zwei.
After a few minutes, Ruby put away her box of memories as she then had the red rose shaped energy appear to bloom from her eye as she then became enveloped in a red light as she became the tall and cool skeleton once more.
"Bye Yang, gotta go help out with the date. You know how these True Pacifist runs work." Ruby shouted in her once again male voice.
Yang chuckled as she then teleported into her purposely messy room before pulling a box of her own memories that had...
Two golden bracelets.
The same outfit she wore in the photo.
Then the same photo that Ruby has.
But then they're was Ruby in her battle body, Yang in her current outfit, and a woman that looked like an older Ruby that wore lab coat. Yang and Ruby knew who she was. After all...
...she was Ruby's biological mom and Yang's stepmother Summer Rose.
Yang then pulled the photo of them and Summer and smiled. She then decided to turn back into her short chubby skeletal form as she goes to Grillby's.
After she ordered takeout (with yet another helpful reminder from Grillby to "Pay her F#%ing tab.") she takes the food to a lab hidden behind the house as she sees Summer who was working on a project.
"Hey mom. I got you some Grillby's." Yang said as she handed her the bag which held a burger with a side of fries and a side of a lemon-lime drink.
Summer looks at her stepdaughter and smiles as she said "Thank you," before hugging her.
Yang smiled as she left Summer to her privacy and work as she went to the Judgement hall to go help out Ruby as she is "tricked" by Flowey to gather everyone together and distract the king of all monster known as Asgore.
She and Ruby grew aware at the many runs Frisk has done by this point that they've all seen where they lead. Due to being injected with Determination as an experiment by Summer, who also was injected with Determination as the three of them were human and Alphys wanted to see what would happen to them, would they end up like the Amalgamates?
Thankfully they didn't die but instead found the power to remember Resets,even TRUE Resets could not touch them. They also grew magical attack power that resembled bones and Skull shaped cannons, they also became Telepathic and move things with little concentration.
They've seen Genocide routes, Pacifist routes, and Neutral routes, they remember and try hard to pretend they didn't happen. Yang remembered fighting Frisk when they ended every single life they could end in a Genocide run.
When she got to the Barrier to see the others waiting for her as the rest of the events played out and then, as usual, that demonic flower appeared.
Yang and Ruby awaited Flowey to absorb them and he did.
Flowey was always too distracted in the monster SOUL buffet to notice their human SOULS.
After Frisk awoke, Frisk then left for a few hours until they returned, satisfied and crossed the barrier all of their monster friends followed.
When they got there Yang and Ruby began to whisper to each other which did not go unnoticed by everyone.
When they tried to listen Yang and Ruby summoned a wall of bones around them so they couldn't hear them.
After a few minutes Yang and Ruby stopped and put down the wall of bones.
"Huma-Frisk. You asked where me and my brother came from earlier did you not? Can you promise me and my brother something and let us fully explain ourselves?" Ruby asked.
Frisk looked at them confused and nodded slowly.
"Alright but you got to promise us something first. No more resets please." Yang said.
Frisk looked confused but sighed "Why?"
"Because Frisk, we remember all of your resets even your TRUE RESETS." Ruby said.
Frisk looked like they were about to have a heart attack at that little tidbit. The guilt washing over them as they nodded "Yes."
Alphys knew where they two were going with this and looked at them.
"Are you guys really gonna go through with this? Shouldn't you go get HER if your about to do what I think you about to do?"
Undyne looked at Sans/Yang, Papyrus/Ruby, and Alphys. "Wh-what are you guys ta-talking about?" Undyne asked, confusion and fear in her voice. She pulled out a spear in defense just in case this goes south and was some kind Coup.
Asgore also armed himself incase the same scenario happens as well.
Ruby then looked at the scientist "I think we're ready Alphys. Just give us a minute. Sans?"
Ruby and Yang smirked at each other "Care to the honors Yang?"
"Yang?" Everyone but Alphys asked to themselves.
Yang's left eye began to glow a bright cyan flame. Frisk then shuddered at the eye as the cyan flame turned into a yellow flaming heart. The flame then enveloped her body and she glowed a bright yellow.
Ruby was doing her transformation as well and glowed a bright red as well.
The light died down to reveal the two sisters in their human forms.
Undyne's jaw dropped the farthest with Frisk being a close second, Toriel and Asgore tied though and Undyne and Asgore dropped their weapons.
This was going to be a LONG day.
I hope you enjoyed the first chapter of my newest fanfic. Withered Roses and Quenched Flames.
Incase your confused on who is who. Here's a list.
Yang = Sans
Ruby = Papyrus
Summer = Gaster (except she didn't fall into the CORE like he did in canon)
Zwei = The Annoying Dog/Toby Fox
This was an AU where during the fall of Beacon, Ruby and Yang died and were sent back in time (Like Bardock was in Dragon Ball: Episode of Bardock) alongside Zwei. They then woke up in the Underground.
I would continue but those are spoilers for next chapter.
Anyway enjoy and Happy Holidays.