Chara

 refers to the first human child to ever fall past the Barrier on Mount Ebott into the Underground. This person is in fact who the player is naming at the start of the game, and not the controllable character who is played throughout the entirety of Undertale.

Description
Chara is said to have looked incredibly similar to Frisk, down to a "similar fashion sense." They are of ambiguous gender and appear to have also been around the same age as Frisk when they fell into the Underground.

When revealed in the Genocide Route, Chara is seen to be wearing a similar but differently-colored striped shirt, and has rosy cheeks, open eyes, and a vacant smile.

Personality
While initially cast in a sympathetic light by Asriel and others (primarily while they are talking to Frisk but using Chara's name), Asriel admits after being defeated in the Pacifist Route that the reason Chara climbed Mt. Ebott wasn't "very happy," and that they hated humanity, though they never explained why. He goes on to say that Chara "wasn't really a great person." It is hinted that Chara took pleasure in inflicting pain onto others, such as when Asriel mentions that Chara "laughed it off" when they accidentally made Asgore ill by putting buttercups instead of cups of butter into a butterscotch pie they made for him.

History
This first human fell into the Underground after the war between humans and monsters and was taken in by Toriel and Asgore as a second child, whom they treated with respect equal to their biological son, Asriel. Asriel quickly became friends with the fallen child, and grew to respect them greatly. Monsters of the Underground spoke of how Chara filled them all with hope. One day, Chara became very ill, and requested to see the flowers of their home village once more. But because no one could cross the barrier, the request could not be fulfilled. Chara died the next day, and Asriel, wracked with grief, absorbed Chara's soul and became powerful enough to cross the barrier. He took Chara's body and carried it to a bed of flowers in the center of Chara's village. The villagers, believing Asriel had killed the human, began attacking him with everything they had. Although he had the power to destroy them all, he did not fight back; he smiled and carried the human back to the castle, where he himself died from his wounds. Chara was originally laid to rest in a coffin in the basement of the castle, but Flowey implies that Toriel carried the body to the Ruins with her to give them a proper burial.

Several points in the game, such as the VHS collection in the True Lab, imply that Chara's death was part of a plan that they had conceived: to poison themself with flowers, then for Asriel to combine their soul with his to cross the barrier, then to collect six additional human souls to gain enough power to break the barrier (though whether Chara's intention was benevolent is unclear; in fact, it could be inferred that the true plan was to restart the war between humans and monsters). Asriel explains to Frisk at the end of the Pacifist Route that when his soul combined with Chara's, control over Asriel's body was split between them, and that it was Chara who carried their own body to the village. Chara also wanted to use their full power against the villagers, but Asriel resisted their influence.

Main Story
The intro sequence depicts Chara exploring Mount Ebott and falling into the Ruins.

In all routes, if the player gets a Game Over then they will (in almost every case) hear Asgore urging to 'stay determined!'

Neutral Route
The protagonist may hear 's name in their dreams.

When walking through New Home the protagonist is told the history of and Asriel. Before the Throne Room, there is a room with coffins for the seven humans that came before the protagonist, and one of the coffins is for.

Pacifist Route
After saving Asriel, Frisk may return to the coffin room. They see that 's coffin is empty, and notice mummy wrappings at the coffin's bottom.

Genocide Route
The only instance in the game where Chara makes an appearance is during and after a genocide run, where the character plays an important yet somewhat ambiguous role.

It is implied that Chara takes possession of Frisk's soul, and steadily grows in power by killing all of the monsters they can. Although almost everyone in the game recognizes the cold and malevolent behavior of Frisk in this route, only Flowey identifies Frisk with Chara, speaking to them like they have been friends for a long time, until Flowey realizes that he too may be killed.

After completing the first genocide run, Chara will suddenly appear to address the player directly, saying they were brought back to life by the player's power and determination, and that the purpose of their reincarnation is power itself. They go on to identify themself as the very feeling the player receives whenever their stats increase, and implore the player erase the world so they can move on to the next.

The player is given a choice to 'ERASE' the world, or 'DO NOT'. Regardless of the player's choice, will attack the screen, destroying the world.
 * If the player chooses 'ERASE', will call the player 'a great partner'.
 * Choosing 'DO NOT' will cause a jumpscare, where tells the player that they were never in control, coming closer towards the screen with a horrifying facial expression while apparently laughing as the screen shakes and flashes to red and black.

speaks to the player if they open the game after a genocide run and leave the game open for 10 minutes. They sense that the player wants to go back to the game's world, and blame the player for causing the world's destruction. They offer to restore the world in exchange for the player's soul. Agreeing to this trade will restore the world, deleting the save file, but the pacifist ending will be permanently affected. Refusing will force the player to wait another 10 minutes, after which will offer the same choice again.

After completing the second genocide run, Chara calls themself "the demon that comes when people call its name." They say that the reason the player continues to recreate and destroy the world is due to a "perverted sentimentality" they cannot understand, before suggesting that the player take a different path if they choose to create the world again. still gives the player the 'choice' to erase the world, though it has the same results as the first time on the genocide run.
 * If the player chooses 'ERASE', will once again call the player 'a great partner', saying they will be together forever.
 * However choosing 'DO NOT' again will have tell the player that these very twisted feelings were exactly what it was just talking about. Chara then reminds the player they sold their soul a long time ago, causing another jumpscare, as they destroy the world again.

also appears briefly if Pacifist run is completed after Genocide run, as (without file editing) they will have the player's soul at that point:
 * If the player stayed with Toriel, then the after-credits scene continues briefly after Toriel closes the door, showing in place of Frisk turning to face the screen with red eyes. The game cuts to black with laughter similar to Flowey's, but it is slowed down and lower-pitched.
 * If the player had 'places to go', then the photo will have everyone's faces except 's crossed out with red crosses., who is in place of Frisk, does not have their face crossed out.

Trivia

 * The creator of the game, Toby Fox, made a post on Twitter suggesting that you are supposed to name the fallen child with your own name.
 * The name 'Chara' is used in-game and in promotional material regarding the game:
 * Naming the fallen child 'Chara' results in the game saying that this is 'the true name.'
 * Removing the name from a save file results in the name 'Chara' being given by default.
 * Two promotional gameplay screenshots have 'Chara' as the name. Other materials leave the name blank
 * Whether a coincidence or some other reason, Chara's soul is the same color as Frisk's, which can be seen on the empty coffin in the room down the hall from the throne room.
 * The word Chara (χαρά, ᾶς, ἡ) in Greek means "joy, delight or gladness". In Brazilian Portuguese, the word "Xará" (commonly misspelled as "Chara") means someone with the same name as yours.
 * "Chara" may also be short for "Character".
 * There is a full set of Chara sprites in the game data, including walking animations, puddle reflections, and falls; however, it is uncertain where they are intended to appear.