Chara

 is the first human 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.

Personality
While initially cast in a sympathetic light, Asriel   admits at the end of the Pacifist Route that they weren't the greatest person. Asriel says the reason Satan climbed Mt. Ebott wasn't very happy, that they hated humanity; though they never explained why, they felt very strongly about it. It is mentioned that Satan "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. Asriel states that it was Satan who took control of Asriel's body, picked up their own corpse and carried it out to a human village. He recollects that when the humans attacked them, Satan wanted to use Asriel's full power to kill them.

The reincarnated Satan is cold and malevolent, in clear juxtaposition to their appearance and apparent age. Their speech is slow and silent. They are shown to display curiosity if the player chooses not to destroy the world.

Appearance
Satan is said to have looked incredibly similar to Frisk, down to a "similar fashion sense". Like Frisk, they are of ambiguous gender and appear to have also been around the same age as Frisk when they fell into the Underground. Satan's soul is also the same color as Frisk's.

At the end of the Genocide Route, Satan is seen to be wearing a green and yellow striped shirt, brown pants and shoes. Compared to Frisk, Satan is of a lighter skin color. They have rosy cheeks, open eyes and a vacant smile, displaying an unusally sunny disposition in contrast to Frisk's constant stoic expression.

Main Story
After falling into the Underground, Chara was taken in by Toriel and Asgore as a second child, who treated them with respect equal to their biological son, Asriel. Chara and Asriel became best friends. Monsters speak of how Chara filled the Underground with hope.

One day, Chara became terminally ill, their last wish being to see the Golden Flowers of their village. When they died, Asriel absorbed Chara's soul, and carried their body across the barrier to their village, where he was attacked by its inhabitants. This is later revealed to be in fact a plan by Chara: to poison themself with flowers, have Asriel absorb their soul, and to collect human souls to destroy the barrier. Whether their intentions to conceive such a plan were benevolent or not is unclear; Asriel mentions it was Chara who wanted to destroy the villagers when he absorbed their soul, and Asriel who held back and walked away.

Chara's body was originally laid to rest in a coffin in the basement of the castle, where the bodies of the other humans would later be put. Toriel later carried Chara's body to the Ruins to give them a proper burial.

Neutral Route
When the player gets a Game Over, Asgore's voice can be heard, urging them to "stay determined". The messages heard at this point are identical to those Asgore said to Chara when they were sick and bedridden.

The protagonist may also hear 's name in their dreams. At a certain point in the story, if the protagonist sleeps in Toriel's home, Asgore's voice can be heard, pleading "Chara, please... Wake up! You are the future of humans and monsters." When the protagonist first falls into the Garbage Dump, Asriel's voice can be heard talking to Chara as well.

True Pacifist Route
In the True Lab, VHS tapes of Chara interacting with their family can be played back. Towards the end of the fight against Asriel, when the protagonist calls Asriel's name to save him, flashbacks of Chara play, showing them after they fell into the Underground, being found by Asriel, and with their family. After the fight, Asriel finally recognizes the protagonist as Frisk.

During the epilogue, Frisk may return to the room containing the coffins of the other humans. They see that 's coffin is empty, and notice mummy wrappings at the coffin's bottom.

Genocide Route
Throughout the route, many characters comment on the protagonist's cold attitude, and fail to even recognize them as human. The only instance in the game where Chara makes an in-person physical appearance is at the end of the Genocide Route. After completing the route for the first time, Chara appears to address the player directly.

Chara states they were brought back to life by the player's power and determination, and that the purpose of their reincarnation was power itself. They identify themself as the very feeling the player receives whenever their stats increase, and proposes the player to erase the world so they can move on to the next.

The player is given the choices to "ERASE" the world, or "DO NOT". Regardless of the player's choice, attacks the screen, destroying the world and closing the game window.(because the game had crashed)
 * If the player chooses "ERASE", will call the player "a great partner".
 * If the player chooses "DO NOT", tells the player that they were never in control. Afterwards there will be a jumpscare (watch at the viewer's discretion), where Chara approaches the screen with a horrifying facial expression, the screen shakes and flashes red and black.

After completing the route, will speak to the player if they leave the game open for 10 minutes. They sense that the player wants to go back to the game's world, and remind the player that their actions caused its destruction. After asking the player if they think they are above consequences, Chara offers to restore the world in exchange for the player's soul. Agreeing to this trade will restore the world, but permanently affect the True Pacifist ending. Refusing will force the player to wait another 10 minutes, only for to offer the same choice again.

After completing the route a second time, 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 a "perverted sentimentality" they can no longer understand, before suggesting that the player take a different path if they choose to create the world again. gives the player the "choice" to erase the world again.
 * If the player chooses "ERASE", will once again call the player "a great partner", saying they will be together forever.
 * Choosing "DO NOT" will have tell the player that these very twisted feelings were exactly what they were 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.

Asriel/Flowey
When Chara first fell into the Underground, Asriel found them and took them back to his parents, Toriel and Asgore. Chara seemed to endlessly bully Asriel while they were alive, resulting in Asriel's fear of Chara. This was part of Chara's plan, as they wanted to merge with Asriel's soul after they died to get the six human souls. Asriel states, as Flowey, that Chara was the only one who understood him, and states in the Genocide Route that they would never give him any "worthless pity".

If the player completes the Neutral Run a second time, Flowey says that he couldn't truly care for Chara.

The Player
Chara only reveals themselves if the protagonist completes a Genocide Route. However, they don't seem to talk to Frisk (As some speculate) but rather the player themselves. Chara explains that due to the player's determination, Chara was reincarnated. As the protagonist kills monsters and gains more and more EXP (Under the control of the player) Chara seems to grow more and more strong. Chara refers to the Player as their "partner" if the player accepts Chara's offer to erase the world.

If the player is to complete a Genocide Route a second time, Chara questions why they continuously recreate and destroy the world. Chara then goes on to say that the Player has a "perverted sentimentality" that drives this odd behavior. Regardless of Chara's opinion, they further recreate the world.

Toriel
When Chara fell into the Underground, Toriel acted as their mother. She cared for Chara very much, equally as much as Asriel.

Asgore
Similarly to Toriel, Asgore acted as the parent of Chara and treated them as equally as Asriel. It can be speculated that Asgore's "Mr. Dad Guy" sweater was made by Chara, due to their response to checking it while in New Home.

Chara In-Game
It is theorized that, as opposed to playing as Frisk, the player controls Chara inhabiting Frisk's body, who grows in power the more monsters they kill. It can be guessed that Chara starts to awaken when the kill counter in the Ruins is exhausted, and save points begin to say "Determination". At this point the protagonist begins to display an unseen cold and malevolent nature. This behavior persists until the protagonist shows mercy or resets the game.

In several moments throughout the Genocide Route, Chara temporarily disables the player's input and takes direct control of the protagonist. The first instance of this is during the first encounter and puzzle with Papyrus, where they seem to ignore him and move forward on their own. The last instance of this is at the Throne Room, where without any input from the player, Chara slays both Asgore and Flowey without hesitation. Furthermore, Chara's voice and thoughts start to narrate the text that the player sees. These narrations are sometimes but not always red in color:

appears briefly after the True Pacifist Ending Credits if a Genocide run has been completed beforehand. If the protagonist chose to stay with Toriel, the after-credits scene continues briefly after Toriel closes the door, showing in place of Frisk turning their face to the screen with red eyes. The game cuts to black, with laughter similar to Flowey's, but slowed down and lower-pitched. Similarly, if the protagonist says they have "places to go", the photograph after the credits will have the face of everyone crossed out, except for Chara, who replaces Frisk.
 * Examining Toriel's Home:
 * Kitchen: " Where are the knives. "
 * Mirror: "It's me, Chara."
 * Examining various objects in Snowdin Forest:
 * Hole: There's a hole here.
 * Snowdecahedron: (It's a snow ball.)"
 * Examining various objects throughout Asgore's Home.
 * Padlocks: " (He leaves them in the kitchen and the hallway.) "
 * With one key: " (There are two keys.) "
 * Upon unlocking it: " (I unlocked the chain.) "
 * Calendar: " (The date I came here.) "
 * Beds: "My Bed." and "His Bed."
 * Bookshelves/Asgore's Closet: " Nothing useful. "
 * Refrigerator: " No chocolate. "
 * Kitchen Note: "I've read this already."
 * Stove: "Stovetop."
 * Photograph: " ... "
 * Journal: " The entries are always the same. "
 * Drawing of Flower: " My drawing. "
 * Closet: "Our clothes."
 * Asgore's Dresser: " Still has that sweater. " (Refers to Asgore's pink hand-knit "Mr. Dad Guy" sweater)
 * Mirror: "It's me, Chara."
 * Real Knife:
 * Upon equipping it: "About time."
 * Flavor description: "Here we are!"
 * The Locket:
 * Upon equipping it: "Right where it belongs."
 * Flavor description: "You can feel it beating."
 * When examining the coffin with the red soul on top: " It's as comfortable as it looks. "
 * Several boss fights, as well as some miniboss encounters, show a significant change in tone when the player "checks" the enemy in battle as well.

Name
"Chara" is likely just short for "Character," but there are speculations of its greater signifigance. For example, The word "chara" (χαρά) in Greek means "joy", "delight", or "gladness". When attempting to name the fallen child "Chara", the response becomes "The true name". Removing the name from a save file results in the name "Chara" being given by default. The child being "fallen" can refer both to the child having literally fallen into the Underground, or to the phrase "to fall from grace" (to become morally corrupt).

Trivia

 * Toby Fox suggests to name Fallen Child the player's own name.
 * Chara is implied to like chocolate; should the protagonist check the refrigerator in Asgore's Home, the text reads " No chocolate. ".
 * While there is no chocolate in Asgore's home, there is a bar of chocolate in the fridge at Toriel's; it is possible Toriel kept it in her fridge in memory of Chara's tastes.
 * The Heart Locket most likely belongs to Chara, since the text "right where it belongs" is shown when the protagonist equips The Locket.
 * Within the game files, there are sprites of Frisk wearing Chara's shirt and pants, lacking a face but having shading under the eyes.