Frisk

 is the playable character, whose name is revealed only during the ending of the Pacifist Route. They are one of Eight humans to pass through the Barrier into the Underground by way of Mount Ebott. Undertale documents Frisk's journey and attempt to escape the Underground.

Appearance
Frisk is depicted as a young child of ambiguous gender and ethnicity, with medium length straight brown hair and a blank expression. They wear a striped purple shirt, short pants, and plain shoes.

While the game always shows Frisk with a blank expression, other characters and flavor text describe them making other expressions. For example, when Frisk cheers Napstablook in their battle, they are described as giving "a patient smile."

After you play a Genocide Run and complete a True Pacifist Run, Frisk's appearance in the end cutscene--with Toriel handing them pie as they sleep--will change to having greener skin, red eyes, and cheek markings. This appearance is a result of Chara possession over Frisk´s SOUL.

Personality
Frisk talks to other characters, but unless the player receives a prompt on what to say, their dialogue is not shown. During battles, the flavor text describes what Frisk says. Outside of battle, when Frisk speaks on their own, the other character's dialogue box displays "..." to indicate them listening to Frisk.

Depending on the ending that the player is moving towards, Frisk can be depicted as friendly, in the case of the Neutral (or especially the Pacifist Route). In the Pacifist Ending, characters such as Asriel make emphasized note about Frisk's kindness, understanding, and grace. This is also the only ending when Frisk's name is given, which implies that this is their true character and personality.

In the case that the player chooses the Genocide Route, Frisk comes off as cold, eerie, and sinister to all that meet them. Towns are evacuated and monsters are recommended to not interact with Frisk in any way possible. Flowey and Sans perceive the character as not being human (in the sense that they lack any humanity) and Asgore does not recognize them as human (in other playthroughs, he immediately recognizes them.) It is, however, unclear how much of Frisk even remains at this point: many interactions with the environment, starting from Toriel's house, suggest that on this route Frisk is, in fact possessed by the First Human.

Loading saves or resetting will not affect Frisk's memory of certain events. Some characters notice Frisk behaving strangely as a result of this.

It should be noted Frisk is designed for the player to put themselves in the shoes of Frisk. Therefore, their personality is not shown very well in the game by canon alone, and is truly up to the interpretation of the player.

Abilities
Frisk has a great deal of determination, allowing them to save and heal at save points. When Frisk has a save file, they can load it from the menu screen, and will load during the game if they die. Frisk can also reset the world to when they landed in the underground, though this is not enough to completely erase other's memories of what happened before the reset.

Main Story
Little is known about Frisk's life on the Surface. Even less so their motive for entering the Underworld. What is certain from one of the endings, is that the child has no parents or family to call their own.

Frisk is the eighth human to have entered the Underground. They fell on a plot of golden flowers.

Neutral Route
Having landed in the Ruins, Frisk meets Flowey and Toriel. Toriel guides them through the ruins, giving them a cell phone on the way to her home, intending for Frisk to stay with her. However, Frisk asks about leaving, leading Toriel to attempt to destroy the the Ruins' exit to protect them from Asgore. Frisk either manages to convince Toriel to allow them to leave the Ruins, or kills her.

Journey
Entering Snowdin Forest, Frisk meets skeleton brothers Sans and Papyrus, who are sentries looking for humans. Sans doesn't care about capturing Frisk and assures them that Papyrus is harmless, making them less fearful of the underground's inhabitants. Frisk also encounters guard dogs, but they get past them. Entering Snowdin, Frisk meets some friendly monsters who assume them to be another monster, and they have the opportunity to learn a little about monsters in the library. Papyrus tries to capture Frisk, but he relents or is killed. If spared, Papyrus tells Frisk that the exit is in Asgore's castle.

In the Waterfall, Frisk makes friends with a monster kid while avoiding the captain of the Royal Guard, Undyne. Frisk learns more about monsters through old writings in the Waterfall. They separate after the kid learns that Frisk is a human. Frisk learns from Undyne that Asgore wants to take their soul, as their human soul is the last one needed to break the barrier keeping the monsters trapped underground. Undyne tries to kill Frisk but they either kill her or escape to the Hotland, where she collapses from overheating and exhaustion.

In the Hotland, the scientist Alphys attempts to befriend Frisk in her lab with the help of her friend and human-killing robot Mettaton. Mettaton tells Frisk the truth at the exit to the Core, and attempts to take Frisk's soul to escape to the surface himself and prevent Asgore from destroying the barrier. Frisk kills Mettaton or they help remind him of his importance to the monsters in the underground, convincing him to stay and let Frisk leave. Alphys informs Frisk that they will need to take a monster's soul to leave, and that they must kill Asgore for this.

Reaching the capital city of the underground named New Home, Frisk learns the story of Asgore and Toriel's children, Asriel and the first human, from monsters passing by. It is evident to Frisk that the monsters are filled with hope from Asgore's plan for freedom. In a corridor, Sans judges Frisk for the monsters they have (or have not) killed, explaining the meaning of LOVE and EXP and telling them that they will decide the fate of the world.

Escape
Frisk enters the Throne Room and finds Asgore. Asgore tries to make small talk, but they remain silent. The two walk to the barrier, and Asgore gives Frisk an opportunity to back out if not ready. Eventually, when they are ready, Asgore shows the six collected human souls to Frisk before they fight. Asgore temporarily prevents Frisk from showing any mercy to him, but after Frisk defeats Asgore, they regain the opportunity to spare him, though they can kill him instead. Flowey suddenly appears, destroying Asgore's soul, taking the human souls and closing the game.

The player reopens the game, only to find that Flowey has saved his own game, having gained the power to do so with his stolen human souls. Loading Flowey's game, Frisk confronts Flowey and defiantly steps forward, entering an initially hopeless battle with him. Frisk calls out for help and the stolen souls begin to revolt: first they heal Frisk, then they weaken Flowey, leaving him vulnerable and eventually removing his ability to save or load. The souls leave Frisk and a weakened Flowey alone, and Frisk has the choice of sparing or killing Flowey before they leave the underground. It is not explained how Frisk passes through the barrier, but it remains active and keeps the monsters trapped.

Some time after Frisk escapes, Sans calls their phone, leaving a message about what happened to the underground after Asgore died. None of the monsters know Frisk's name.

Pacifist Route
Dissatisfied with the neutral ending of their journey, Frisk loads their save file, or they reset the world. Frisk makes sure that they don't kill anyone.

Friendships
Frisk gains Sans' friendship by sparing Papyrus. They visit Papyrus at his home, and hang out or go on a date with him, gaining his friendship.

When Frisk was chased by Undyne, who then collapsed, they gave her some water, and she returned to her home. Frisk visits Undyne with Papyrus' help, and Undyne gets 'revenge' by trying to befriend Frisk, setting her house on fire in the process. She tries to fight Frisk to regain honor, but Undyne realizes that Frisk is a 'wimpy loser with a big heart', just like Asgore, and decides to become their friend.

Undyne, having been encouraged to send a love letter to Alphys, gives the letter to Frisk who delivers it to Alphys. Alphys goes to the garbage dump and is encouraged to tell Undyne about her feelings for her, gaining Frisk's friendship. Alphys tells Undyne that anime is not real, among several other truths, and Undyne asks Frisk if anime is real before leaving.

After befriending Alphys, Papyrus recommends that Frisk go to the lab. There, they find a note from Alphys and enter the True Lab to support her. She thanks Frisk for coming and gains the confidence to reveal the truth about her experiments to the rest of the underground. Frisk attempts to leave the true lab but they receive a phone call from a mysterious voice, who mistakes Frisk for the first human, before they are forced into New Home.

Barrier
Confronting Asgore again, Frisk and Asgore are stopped from fighting by the former's friends. Flowey appears, having stolen the humans' souls, trapping the monsters and trying to kill Frisk, but their friends use their magic to protect them. All of the underground's monsters arrive, falling into Flowey's trap of absorbing their souls. Only Frisk's soul is left.

Flowey reveals himself to be Asriel, regaining his own form with the power of everybody's souls. He fights Frisk, turning out to have been the mysterious voice in the True Lab, and he continues to mistake Frisk for the first human. Frisk, unwilling to fight Asriel, holds on to their hopes and dreams and refuses to die, preventing Asriel from gaining enough control over the timeline to reset it. They are trapped by Asriel, and realize that all they can do is save the monsters' absorbed souls. Saving their friends' souls by calling out their names and talking to them, regaining their memories, Frisk identifies one more friend to save, Asriel, and calls his name. Frisk sees Asriel's memories of the first human, who looked much like Frisk themself. Asriel starts to feel emotions from the monsters' souls, and eventually stops fighting, telling Frisk that he's afraid and alone.

Asriel, having realized that they are not the first human, learns Frisk's name. He apologizes for hurting everybody, and Frisk has the option to forgive him. Asriel destroys the barrier with the souls before releasing them, and knowing that he will turn back to a flower, begins to leave; Frisk has the option to comfort him.

Freedom
Frisk wakes up with their friends (Toriel, Sans, Papyrus, Undyne, Alphys and Asgore). While the monsters remember little about Flowey and nothing about Asriel, they do know Frisk's name. Frisk gets the opportunity to go through the underground and talk to the other monsters, including Asriel.

Frisk leaves the underground with their friends. Asgore offers them to be the monsters' ambassdor to the humans. Most of Frisk's friends then leave, except for Toriel, who asks Frisk what they will do now, and Frisk has the choice of living with Toriel or not.

If Frisk chose to stay with Toriel, the ending scene shows Toriel bringing a slice of pie to Frisk's bedroom. Otherwise, the game ends with a group photo of Frisk and their friends.

Trivia

 * If the player chooses the name "Frisk" during the beginning of the game, they will be put into "hard mode" which only lasts the length of the demo, but uses significantly harder enemies (with some from the Core and some Hard Mode only Monsters), ending at the exit to the Ruins.
 * Frisk is allergic to Temmie, but seems unaffected by other furry residents of the underground.
 * It is possible that Frisk falling into the Underground was an attempt of suicide, though this is only hinted at, because of the conversation where Frisk was asked why did they climb the mountain: "Was it foolishness? Was it fate? Or was it... because you...? Well. Only you know the answer, don't you..?" However, another interpretation of the question could be that it's just an open question to the player to reflect upon the reason of playing a game, as "entering the underground" can be interpreted as the act of entering a game world, i.e. Chara entered the underground because they were "not happy" in real life and just wanted to gain immense power and destroy everything while in the game world.
 * "Frisk" is the Swedish word for "healthy".
 * "Frisk" is similarly the danish word for "fresh"