I am Lucida, I am the sister of Sans and Papyrus. Not many people know my identity, but I like that. After Frisk freed the Underground from the Barrier, I still kept myself a secret. Anyway, here is my story:
Years ago, a human fell into the Underground, her name was Chara. She was originally greeted by Prince Asriel Dreemurr, and became the adopted Princess of the Royal Family. She acted like a nice person when she was around her new family, but I watched her when she was alone at night. She was a murderer... she would kill every monster she came in contact with. Every morning though, before Asriel arose from his slumber, she would clean her knife and put it back in the kitchen, then clean the dust off her clothes. One day, she as far as to "accidentally" poison King Asgore by putting buttercups in his tea.
I had to end this, so one night, I confronted her.
"You've killed many monsters, Princess Chara. I hate to do this, but you have to die." I said to her.
"Well now, this will be interesting." She said with a smile on her face.
"We have a comedian, and a megalomaniac. I thought Sans was supposed to be the comedian of your family."
"Where do you think he gets his puns? It kills me to try to teach him." I say, and pull out my trident and slash at her.
I hit her directly, leaving a cut on her face. She wipes off the blood and rushes toward me. I teleported away from her blade and put a spell on her that caused her to become terminally ill. She was knocked out from the spell and I teleported her to her bed in the Royal Home.
The next day she died. Asriel was devastated and decided to absorb her SOUL, which gave him enough power to cross the barrier and carry Chara to her home village. She was never seen again for years.
After the death of Chara, the Underground was at peace again. Though another human fell into the Underground, a pacifist. Frisk. She freed the Underground and destroyed the barrier after Asriel was reborn.
Though after she freed us, she reset the timeline and became genocidal. She killed everyone; Toriel, Papyrus, Captain Undyne, even Asriel himself as a flower. I finally possessed Sans and battled her. It was at that point that I realized that Chara's SOUL was living inside her. Possessing Sans destroyed my physical form, so when she eventually killed him, then the King. Because my physical form was destroyed, I was there to witness Chara destroy the world, then reform it.
That is my story, but I found recently found a message from my father: W.D. Gaster. He must have written it knowing I would read it to someone other than Sans and Papyrus. Here it is: ANY HUMAN THAT IS TO FALL INTO THE UNDERGROUND WILL HAVE SIMILARITIES, THEY WILL BE CURIOUS OF OUR WORLD, THEY WILL FIND SO CALLED EASTER EGGS. THEY WILL ALSO TRY DIFFERENT ENDINGS, THIS CANNOT HAPPEN. THEY MUST REMAIN ON THE PACIFIST ENDING, LEST THE WHOLE UNDERGROUND START TO GLITCH. I will not decipher this, but this is a serious warning. Follow it, and stay Determined.