Just to let you all know, this story has nothing to do with the other Undertale I'm working on, this is a bit from another person on Tumblr, go check it out, it's pretty good; please enjoy.
The story starts as it always has, our main protagonist, Frisk, falling into the Underground, but what they didn't realize is that they awakened someone. "W-What happened?" they looked at their hands "what? Impossible, our plan, failed". This person was a spirit dressed in a green sweater with a yellow line.
The person that fell stood up as the spirit looked in shock. The human rubbed their head, looking up, seeing where they fell into. They went off to the next room as the spirit followed. The human ran into a little flower "Howdy! I'm Flowey! Flowey, the flower".
Later, after Toriel dealt with Flowey, she led the human through the ruins as Chara followed confused. "Wait, mom, what's going on here? Where's Dad, where's As-" they reached out but faded through her "w-what?". "The ruins are full of puzzles, ancient fusions between diversions and door keys". "One must solve them to move from room to room, please adjust yourself to the sight of them".
The spirit looked around the catacombs "hmm, it all looks the same as before, how long have I been asleep?". The spirit saw the human looking at them, they looked around "can you, see me?" the human nodded "… perfect".
The human looked at the sign "can you read that?" "no" they looked down. "Okay, I'll help; it says, Only the fearless may proceed. Brave ones, foolish ones. Both not walk the middle road". They continued to the dummy room "when you encounter a monster, you will enter a fight". "strike up a friendly conversation and I will come to resolve the issue" "that's strange, monsters didn't fight me unless I started it".
After finishing up the other puzzles, they came to the end of the corridor. "Please remain here my child, it's dangerous to explore around here" Toriel gave them a cell phone. "If you need anything, just call" "wait, why does that look like my old cell phone" Toriel left.
The Human was sitting by the column, waiting for Toriel. "You know, it might take a long while until she gets back, you also might starve". "C'mon, let's explore" the human stood as they and Chara left, as they got a call from Toriel "you haven't left the room have you?" as the human got worried "man, she was never this worried before". "By the way, what's your name? I'm Frisk" "why should I? I barely even know you" Frisk sulked "… It's Chara".