UnderKnown is an AU with many different changes from the original timeline. To make this slightly easier to understand, I have listed some general ideas of most of the main characters. WARNING: SPOILERS OF STORY PLOTLINES AHEAD, PROCEED WITH CAUTION. THIS WILL BE CONSTANTLY EDITED AS THE STORY PROGRESSES.

Frisk: A mage who can't use magic. Is the eighth fallen human and the second child to be forced up Mount Ebott because of the prophecy of the Underground going empty. Can see other timelines, but only in dreams and sometimes when they see people they have dreamed about. Dreams mostly consist of nightmare timelines and occur at least once every night. Worse nightmares cause them to scream when they wake up, losing any possibility of speech. Complete Pacifist, most likely because of nightmares. Currently has the ability to SAVE, CONTINUE, and RESET.

Asriel: Crown Prince of the Underground, son of King Asgore and Queen Toriel. Works with his mother in the Ruins to help fallen humans find new homes in the Underground. Has some dreams/nightmares of other timelines, but usually only glimpses or feelings. Has been told about the other timelines by his adopted human sibling, Chara. Found Chara when they first fell down, and they both soon became best friends.

Toriel: Queen of the Underground, married to Asgore. Is in charge of helping fallen humans find new homes in the Underground, with her son as her apprentice. Suggested adopting Chara into the Dreemurr family when they fell down. By the time the second child had fallen, the royal family had successfully created a way for humans who have fallen to integrate into monster society.

Asgore: King of the Underground, married to Toriel. Is in charge of the Underground. After the Dreemurrs created a way to integrate humans into monster society, that gave him rising responsibilities, and he had found it increasingly difficult to spend personal time with his family. Chara suggested that the rest of the family take some of the duties that had stemmed from the humans' arrival. Toriel would live in the Ruins with Asriel to monitor the falling of any humans, and Chara would stay with Asgore at New Home and handle the official processes. That left Asgore with enough room for his kingly duties, as well as time for family.

Chara: Adopted human child of the King and Queen. Is responsible for officially documenting human adoptions and helping humans integrate into monster families. Former mage. The first fallen human and the first child to be forced up Mount Ebott because of the prophecy of the Underground going empty. Can see other timelines like Frisk, but because they have been underground so long, they can see different layers of reality on one plane of existence. Dreams mostly consist of dream timelines and occur at least once every night. Explained to their brother, Asriel, about other parallel universes. Sightly genocidal, possibly due to seeing so many "happy"(true) endings. First to have the ability of SAVE, CONTINUE, and RESET.

Sans: Brother of Papyrus. Works with his brother as a Sentry, and also has other part-time jobs. Is the only one who remembers all the COMPLETE RESETS. Doesn't experience or know which human CONTINUES, but knows when they happen. Because of this, his sense of time is confused. Assumes the most recent human is the only one who can CONTINUE and RESET. Doesn't completely understand the ability of SAVE, as monsters cannot see SAVE POINTS, only humans can.

Papyrus: Brother of Sans. Works as a Sentry in Snowdin. Doesn't know of RESETS or CONTINUES like his brother. No matter which point in the timeline, he is always worried about his brother's growing depression and laziness. Has adopted Bravery into their family.

Undyne: Head of the royal guard. The point of the royal guard is to protect the royal family and to regularly update Asgore on the state of the humans and the monster families that have adopted them. Since adopting Justice, she has had a more brighter look on humans. When not working, she visits Alphys in her lab.

Alphys: The current royal scientist. Is in charge of finding a way to break the barrier without putting the humans in danger. Perseverance, the most recent fallen human at the time, gave her the idea to try using human SOULS for experimentation. After helping her fix the broken DT Extractor, Perseverance was the first and only human to test it. Having extracted some of the DETERMINATION inside Perseverance successfully, Alphys tried experimenting with "fallen down" monsters, causing the Amalgamates to form. Both Alphys and Perseverance blame themselves for this happening. The rest of the monsters in the Underground don't know of the Amalgamates.

Mettaton: The most famous (and only) robot in the Underground. Adopted Integrity before he asked Alphys for a body. Since receiving his robot body, Integrity is at times the only one who truly understands him, and appears on his show quite often.

Napstablook: Last remaining Blook of the snail farm. After Hapstablook (aka Mettaton) left the snail farm to become a star, he adopted Patience. The snail farm has been less lonely for him since then.

Muffet: Head of the Spider Bakery. Since adopting Kindness, the prices for her food have become more reasonable. Kindness helps Muffet create new recipes, especially ones that don't include spiders.

The Six Fallen Humans: Like Frisk and Chara, all six humans previously had the ability to SAVE, CONTINUE, and RESET. After a new human falls down, that period in time is the new permanent "COMPLETE RESET point". Only the most recent fallen human has the ability to SAVE, CONTINUE, and RESET, but the ones who don't have this option can only CONTINUE from the current human's SAVE point. Only the human who has CONTINUED can see that they have CONTINUED, and for the others it seems like the same timeline. Each does not know the others have this ability.

Determination: Asgore, Toriel, and Asriel; The royal family

Bravery: Sans and Papyrus; The skeleton brothers

Justice: Undyne; The head of the royal guard

Kindness: Muffet; The spider bakery

Patience: Blooky; The snail farm

Integrity: Mettaton; The fabulous robot superstar

Perseverance: Alphys; The royal scientist

The order of the fallen children is as follows: (1) Determination, (2) Perseverance, (3) Integrity, (4) Patience, (5) Justice, (6) Kindness, (7) Bravery, (8) Determination

Well, it seems like that should be it with most of the characters. If I haven't put any details about a certain characteristic, assume it is cannon-based, or ask me specifically.