Frisk, or what the player chooses to call them, is the equivalent of Undertale's Chara. They were the first of the eight humans to fall into the Underground after climbing Mt. Ebott.
Frisk is a child whose gender is up to interpretation. They appear at the end of the Genocide Route and speak with the player themselves. Their personality and appearance are identical to the protagonist.
Frisk's appearance is very similar to the protagonist.
They are shown wearing a blue, long-sleeved shirt with a pink stripe (which is the inverted color of the protagonist's shirt) and brown pants along with brown shoes. They have medium length, straight brown hair with short bangs. Unlike in Undertale, Frisk is usually seen with their eyes open and a little smile, only to jumpscare you.
While initially cast in a sympathetic light, Monster Kid admits at the end of the True Pacifist Route that Frisk "...wasn't really the greatest person." Monster Kid states that Frisk climbed Mount Ebott for an unhappy reason, and adds that Frisk "hated humanity". Frisk never told Monster Kid the reason that they had such hatred. Monster Kid also mentions that Frisk "laughed it off" when the two of them accidentally poisoned Toriel by putting buttercups instead of cups of butter into a butterscotch pie they made for her.
At the end of the Genocide Route, Frisk behaves in a polite, deliberate manner, thanking the protagonist for guiding them. However, if the protagonist refuses to destroy the world, Frisk immediately becomes malevolent, despite the outward semblance of youth and innocence. Frisk reveals that the protagonist's "Determination" and "human soul" awoke Frisk from death as soon as the protagonist entered the Underground.
After falling into the Underground, Frisk was taken in by Toriel and Asgore as a second child and was treated with respect equal to their other adopted child, Monster Kid. Frisk and Monster Kid became best friends. Monsters spoke of how Frisk filled the Underground with hope.
One day, after becoming terminally ill from consuming buttercups (which were confirmed to be poisonous), Frisk expressed the desire to see the Golden Flowers found in their home village. After their death, their SOUL was absorbed by Monster Kid, and they shared control over a body. Frisk carried their own corpse across the barrier. However, when they got to the village the villagers thought that Monster Kid had killed Frisk. Because of this, the humans attacked them. Frisk wanted to use their full power and destroy all of the villagers. However, Monster Kid resisted Frisk, which ultimately led to the humans killing the fusion of the two SOULs.
Frisk's motives in this situation are uncertain. In the fourth True Lab tape, Monster Kid says "Y...yeah! We'll be strong! We'll free everyone.", and in the fifth "Six, right? We just have to get six..."
In the Genocide Route, Temmie tells Chara that they should finish what they started and free everyone. Temmie also adds, "Then... let's let them see what humanity is REALLY like. That despite it all... This world is still 'kill or be killed!!". Monster Kid tells the protagonist that they came up with this philosophy because of their resistance to Frisk had led to both of them being killed.
At the end of the Genocide Route, Frisk says little about MK's betrayal other than, "At first, I was so confused. Our plan had failed, hadn't it?"
Frisk's body was originally laid to rest in a coffin in the basement of the castle, where the bodies of the other fallen humans would later be put. When Asgore left Toriel, he carried Frisk's body to the Ruins and gave them a proper burial.
Frisk appears in front of the "protagonist" and says that the protagonist's "determination" and "human SOUL" awakened them from death. There is evidence that some narration, as well as descriptions of certain actions and events, is by Frisk themselves. However, the narrator throughout the entirety of Underswap is up to speculation.
When the protagonist gets a Game Over, Toriel's voice can be heard as she urges Frisk to "stay determined." The messages seen on the game over screen are identical to what Toriel said to Frisk while they were on their deathbed.
The protagonist also hears Frisk's name in their dreams. If the protagonist sleeps in the bed in Asgore's home, Toriel's voice can be heard as she pleads, "<Name>, please... Wake up! You are the future of humans and monsters." When the protagonist first falls into the Garbage Dump, MK's first words to Frisk appear on the screen, narrated with their voice bite.
In the True Lab, VHS tapes of Frisk interacting with their family can be played back. None of these VHS tapes include visuals and provide narration alone. Most of these tapes are of Monster Kid and Frisk, and prove that Frisk had a "plan." In one of the tapes, MK tells Frisk to wake up and that "they don't like this plan anymore." MK's quote is speculated to be something that they said to their dying sibling because this same VHS contains the quote seen on the game over screen.
Towards the end of the fight against Monster Kid, when the protagonist calls out "someone else"'s name, flashbacks of Frisk and Monster Kid play. They show Frisk's fall into the Underground, Monster Kid's discovery of Frisk, and the royal family spending time with Frisk. After the fight, Monster Kid recognizes the protagonist as Chara.
During the epilogue, Chara may return to the room containing the coffins of the other humans. This leads to the discovery that all of the coffins are empty and that there are mummy wrappings at the bottom of Frisk's coffin.
While traversing the Genocide Route, many characters comment on the protagonist's creepy stare. Papyrus comments that it would be great if the protagonist continued pretending to be a human, and Toriel inquires what kind of monster the protagonist is.
At the end of the Ruins and when going through New Home, Temmie determines that the protagonist is empty inside, just like herself. She concludes that the protagonist is Frisk, and informs them of a plan she has to become omnipotent.
Frisk seems to be in control of the protagonist's scripted sequences, as the sequences in the Genocide Route differ from the Neutral and True Pacifist Routes. Instead of following other NPCs commands, the protagonist defies them and becomes merciless as the route progresses. The first example of defiance occurs when the protagonist ignores Papyrus's request to hide behind the conveniently shaped lamp; the last is when the protagonist kills Temmie before Frisk makes their appearance.
The only instance in the game where Frisk makes an in-person physical appearance is at the end of the Genocide Route. They appear as an overworld sprite and speak into the fourth wall. Frisk could be speaking directly to the player in this sequence.
After this appearance, Frisk reveals that the protagonist's "human soul" and "determination" were the cause of their reincarnation and that the merciless events that occurred during the route lead Frisk to realize that the purpose of their awakening was power itself. Frisk states they are the embodiment of the feeling received whenever STATS increase and propose erasing the world and moving on to the next.
The protagonist is given the choices to "ERASE" the world, or "DO NOT." Both choices lead to the same result.
- If the protagonist chooses to "ERASE," Frisk calls the protagonist "a great partner." Additionally, their eyes widen, and Frisk says to the protagonist that they will be "together forever."
- If the protagonist chooses "DO NOT," Frisk's eyes widen, and they say that the protagonist was never in control. Afterward, a disturbing jumpscare occurs, in which Frisk approaches the screen with a horrifying laugh. The screen (or game window) shakes while loud droning sound loops play in the background.
After the protagonist makes their choice, an attack animation occurs, followed by red "9"s covering the screen. The game is forced into windowed mode and shakes similarly to a monster's death animation as the game closes. The player cannot exit the game before the crash unless they use the OS's Task Manager.
Upon reopening the game, the screen is black and accompanied by a sound similar to howling wind. After 10 minutes, Frisk speaks to the protagonist, sensing that they want to go back to the game's world, and reminds the protagonist that their actions caused the world's destruction. After asking the protagonist if they think they are above consequences, Frisk offers to restore the world in exchange for something, later revealed to be the protagonist's SOUL.
- If the protagonist agrees to this trade, the world is restored, but the True Pacifist ending is permanently altered.
- Refusing this offer causes Frisk to abandon the protagonist, only to offer the same choice again after the game is restarted and another 10 minutes elapse.
- Frisk is only encountered in the Genocide Route ending.
- If the game is not in full-screen during the jumpscare, the whole window shakes.
- In the Genocide Route, Frisk says that the protagonist's "determination" and "human soul" awakened them from death. There is evidence that some narration, as well as descriptions of certain actions and events, is by Frisk themselves. However, the narrator throughout the entirety of Underswap is up to speculation.