* kid, listen. there are spoilers on this page; be careful when you're reading.
* you don't want a bad time, do ya?
Since the story takes place after a genocide run, she still harbors large amounts of guilt for killing her friends and family.
The story is mostly told from her perspective.
It is unknown exactly what Frisk looks like in this A.U., as it is in the form of written story, and her appearance has not been given. It's only known that she's female and is not quite as tall as Papyrus.
Obviously, Frisk's personality is much more clear in the A.U. than in-game. She has been shown to often hide her feelings and inner demons, as she never revealed the sad story of her life before falling to the Underground to anyone for a long time. She has shown large amounts of guilt for the genocide she caused, and she can't find it in herself to forgive her for it, even when all her friends have. Ever since the genocide run, she has refused to hurt any creature; however whether or not she can keep this up is debatable, as she has gone into a rage when talking to the man who killed her parents. She has also been shown to like puns; she and Sans once had a sort of pun battle (Undyne got really annoyed).
Frisk has shown no sort of abilities aside from the save, load, and reset abilities. However due to waning levels of determination in her, it's questionable whether she'll always have that power.
While Frisk's birthday is not known, with some simple math it can determined she born in either 7459 or 7460 C.E. She is presumably the only child of the poor couple Jonathan and Clara Eden. At the age of 10, both of her parents were killed by Alexander Thompson, the United States of the Americas' representative for the United Nations of Earth. He stated the reason for the murder of Frisk's parents being that they broke international law and conducted experiments with the space-time continuum, which may destroy the entire universe.
Teen and Young Adult Life
Some time after Jonathan and Clara were killed, Frisk was found and sent to an orphanage. Due to mistreatment and severe depression, at age 16, she ran from there with no idea of where she would go. Eventually, a man named Jackson Daldalian found her and brought her to his house, where he and his wife Emily Daldalian took care of her; bringing her back to normal health. However, at some point Jackson developed cancer; he would not survive. Frisk, hating the fact that misfortune came to all those around her, went to Mt. Ebott. She had heard the legends that people who went there never returned; she was determined to kill herself, but didn't have courage to do it. She ended up tripping and falling into the mountain. She merged with Chara, and continued onward; she found the first save point. Her determination to die was her original determination. It would turn into determination to get out of the Underground and save everyone after time went on.
Frisk's first journey through the Kingdom of Monsters was a neutral run. She spared Asgore as well as Flowey (who gave her the in-game deal). Once she and Toriel walked off (where the pacifist route of the game ends), Frisk fell unconscious. Flowey appeared and begged her/Chara to not reset (as he does in-game); after leaving, the options "Continue" and "Reset" appeared. After realizing she couldn't continue, she chose to reset, hating herself for it. She did it time and time again, and eventually, got too angry to do it again. In a fit of anger, she started the genocide route. With Chara's "help," Frisk became more cruel and stronger, and in the end, when the option of erasing the world was given to her, Frisk realized what she'd done, and refused. The same in-game events occurred, and Frisk is knocked unconscious (it would seem any time there's a black screen is Frisk strangely falling unconscious). However, instead of talking with Chara, as it happens normally, W.D. Gaster appeared.
Void and Remake Arc
In the Void and Remake story arc, Frisk finishes the genocide route (the story starts at the death of Sans), and continues on to destroy the world and battle against Gaster, who appears in the void left over. Frisk chooses to spare him, and prevents Chara from taking control of her body to kill Gaster. In the end, Gaster makes her a deal; wanting to atone for her actions, she accepts. Gaster brings the world back and Frisk allows Sans to kill her and take her soul. Frisk does basically nothing except wait for Sans to finish creating a different world after that.
In the new world, Frisk goes through her journey mostly normally up to the Last Corridor. There, Sans tells her his story. Afterwards, they continue into the throne room. After a mishap with Omega Flowey (where Frisk had to load her save file), Asgore, Sans, and Frisk continue to the barrier, where Frisk gives Sans her soul, allowing him to break the barrier. Miraculously, putting her soul back revived Frisk. After, Frisk goes and tells the main monsters in the game about the barrier. She, Papyrus, Undyne, Alphys, Toriel, and a bit later, Asgore, go up to the surface, where the events are very similar to in-game. Frisk eventually goes and gets Sans as well, and the two of them, along with Asgore, Toriel, and two humans go to the Mt. Ebott Research Center. When there, they were asked some questions before sent to a building similar to a hotel (if hotels were free). Shortly after, Sans and Frisk went to an unknown area in Waterfall, where Frisk told Sans her story. Afterwards, they go back to the "hotel."
The relationship between this Frisk and Sans is very... complicated. Frisk thinks Sans still hates her for causing the genocide, and just pretends to be kind; he thinks he still hates her too. Sans can't accept he's forgiven her, therefore he acts like he hasn't, which causes Frisk to feel bad about herself and what she did. Frisk has shown fear towards Sans sometimes, especially in moments when he's lost his mind. Nothing much more can be said with the information the author currently has out.
The original fallen child, Chara, merged with Frisk's soul when Frisk fell on Chara's grave. The determination (not the kind you'd expect her to have) inside Frisk woke Chara's spirit. Upon their meeting, the memories of the other flooded through their head; Frisk described it as feeling weird, and said that Chara had a pained look on her face. Chara's backstory has never been stated as of yet, so it is unknown how painful it was for Frisk. Ever since, Chara observed Frisk's journey, sometimes telepathically communicating or appearing in a sort of "spirit form." This form has been stated to look mostly like a human, except partially see-through; it can be assumed this form floats due to it's lack of physicality. Frisk has stated Chara's something of an observer, unlike the souls, who have been show to be extremely active when part of a monster's soul. Chara's personality has been stated to change with the route done; if Frisk acts cruel, Chara acts crueler. If Frisk acts kind, Chara acts kind, but not quite as much.
Other In-Game Characters
Frisk basically has the same relationship with all the characters as she does in the game; the only differences is that Toriel is canonically her new mother figure, and Asgore recently became Frisk's new father figure. Technically they haven't officially adopted her, but they basically have.