A long, Long time ago when monster's ruled the land, human's were to serve the monster's against their will and be a slave for the whole of their existence. One day, when the King of the monster's, king ASGORE passed on a new law, any human slacking on their job would be sent to an immediate death. The Human's started to plan a revolt in secret while their ruler sent poor and helpless children and parent's to their death before one last law would be passed. Six Children, all chosen by his son. Prince ASRIEL the justice-bringer, Chose The Six Children. Each Representing something. The Six helpless children were brought before the king before being brought to the lab. A place to rip the traitor's apart, The first 5 were fine. Ripped apart and harnessed with ease, but the sixth was different, It caused a malfunction. His adopted daughter was at the 6th's one already to look at the child. Gas and fire started appearing near it before it exploded, the Scientist had rushed INTO the tank, pushing the child outside of the chamber in the lab. Dissapearing like he hadn't existed, while the daughter. CHARA the deathbringer had died from the explosion, the soul from the 6th had been extracted though. The Wife, Queen Toriel the Harmonic knew what her husband had done, leaving the castle forever, lighting it up on fire as she left. Disappearing into the underground world, where she'd be safe from him. The human's had other idea's though, they got Some magic from the now missing queen, sending the monster's down into the same hole they came out of, sealing the wall with thick, fire. Also with magic in it to burn forever, and to also act as a wall, unable to be passed through by any monster. The story was left untouched after, monster's weren't ever seen again on the outer world, forgotten by humanity.
Asgore ~ Ruler of the Underground, burning anyone who opposes him. Heartless and cruel.
Toriel ~ The Biggest Traitor in the history of monster's, causing the Great Overthrow of monster's.
Asriel ~ Sealed in The form of a burned flower, Petal's replaced with fire. Banished from the castle forever.
Undyne ~ The Captain of the Flaming Guard's, Burning all to death who defy their king. Cruel.
Alphys ~ The Fire Scientist, Studying fire and magic. Learning of way's to break the wall, Currently Unsuccessful.
Temmie ~ A small creature. Sell's Weapon's for ash.
Papyrus ~ A weak skeleton, The Flaming Guard's Lapdog, doing task's and their dirty work unknowingly.
Sans ~ A strong Skeleton, Feared by most. His Fire Magic is Unopposed due to having water magic too.
Chara ~ A dead Human, Once the adopted Daughter of King Asgore, Now a stain in their royalty.
Frisk ~ A Human with unnoposed Determination, The Balance of the Blazing Underground in their hand's. They could Save, Destroy or Simply abandon the underground.
Mettaton ~ A weak robot, Burned and misused. Her show is a Show of her Being Burned.
Napstablook ~ A strong ghost, Will not harm you unless you to try pass with force. Can be calmed with Apology.
Genocide ending will result in you fighting Sans and a fraction of Papyrus that Sans saved. The player will have to dodge rapidly and Sans will not give mercy. Papyrus may give mercy if you don't choose fight at all during the battle and only use mercy. He can convince Sans to show you some mercy. If you kill them Chara will appear, giving you the option to 'Burn this World' and start a new one. Or stay in this world and rot away. Choosing to stay will result in her leaving you, removing the 'Reset' option. She will come back one every real world hour, and ask you to reconsider. Choosing to 'Burn' it will trigger a cutscene of the underground burning.
The pacifist route ends with Sans speaking to the player before they move forward on to Undyne, she says the player must pay for their murders(?). She is about to kill you before Toriel stops her, saving you and leaving the underground with you. (editor note, I think you mean a neutral ending?)
True Pacifist end's with Asgore forgiving Toriel for her crimes. Sans's power combined with your Determination and all the monster's in the underground causes the barrier to break. As the monsters leave to find a new land with you, Asriel tries to punish himself by not going. Eventually he relents and comes with them because of him father's forgiveness. Chara's soul will not appear. A Cutcscene will appear with them all looking at the world while their underground burn's.