Chara

"Greetings. I am Chara. Thank you. Your power awakened me from death. My "human soul." My "determination." They were not mine, but YOURS. At first, I was so confused. Our plan had failed, hadn't it? Why was I brought back to life?...You. With your guidance, I realized the purpose of my reincarnation. Power. Together, we eradicated the enemy and became strong. HP. ATK. DEF. GOLD. EXP. LV. Every time a number increases, that feeling... That's me. "Chara." Now. Now, we have reached the absolute. There is nothing left for us here. Let us erase this pointless world, and move on to the next."

- Chara

"Right. You are a great partner. We'll be together forever, won't we?"

- Chara if the player chooses to destroy the world

"Interesting... SINCE WHEN WERE YOU THE ONE IN CONTROL?!"

- Chara if the player chooses not to destroy the world

Chara (last name Dreemurr upon being adopted by Asgore and Toriel) also known as The Fallen/First Child is a posthumous character in the 2015 video game, Undertale, being indirectly responsible for the events of the series and the creation of Flowey. However, they also serve as the hidden main antagonist of the Genocide route if the player decides to kill every monster in existence.

Backstory
For some reason, Chara seemed to despise humanity and its actions, despite apparently being human themselves. As they wandered up a mountain, they tripped on a root and tumbled into the Underground, where the monsters lived their lives after the war against humans.

A monster child by the name of Asriel Dreemurr befriended Chara, and his mother and father Toriel and Asgore took Chara in and raised them as a second child and beloved sibling to Asriel.

It soon became apparent that Chara still wanted revenge on the humans, and forced Asriel into a plan. Upon Asgore being poisoned from Buttercups, Asriel was terrified while Chara started laughing at the event. Chara hatched their plan by desiring to use a soul to destroy the barrier between the Underground and the surface. They fatally poisoned themself using the Buttercups, and after the death of their body, Asriel absorbed Chara's soul. Transforming him into a beast, Asriel and Chara assended up to the surface, intending to destroy all humans. Unintended by both of them was sharing joint control of the body. When it came time to go through with their plan, Asriel refused to go through with it and an angry Chara tried to force him to. Thinking that the child had been killed by Asriel, the terrified humans mortally wounded Asriel, who broke Chara's control and smiled at the humans, showing them that he still forgave them because they weren't thinking straight. Asriel and Chara were buried by their parents, and Asriel's dust (for monsters turn to dust upon passion) sprinkled around the burial.

Due to the experiments of Alphys, the dust around the new flower bed had it's DNA and memories placed into a flower, imbuing it with determination and causing Asriel to be reborn as Flowey, who would eventually become a sociopath similar to the one who had manipulated him.

Chara's spirit remained dead, or so it seemed...

Genocide Route
Chara is the true main antagonist of the Genocide route. Over time, as the player kills every monster in the game, Chara gains more and more control over Frisk. They encounter and defeat every monster in the game, including Toriel, Papyrus (who still believes they can change), Undyne, the Royal Guards, and Mettaton.

At last, only one monster stands in their way of Asgore: Sans, who surprisingly and efficiently proves to be the most powerful character in the game. If Chara manages to strike Sans, he reminds them that he warned them of the consequences before heading off to Grillby's and seemingly dying and dissolving offscreen. Chara (and Frisk) proceed to attack Asgore even when he kindly spoke to them, and then he is killed by a horrified Flowey who reveals that he was always on Chara's side after all and begs to them to don't kill him as he reveals to be Asriel Dreemurr, but Chara doesn't care and completely slashs him from existence. Chara reveals themselves at last and notes that all the power stats represent themselves. They ask the player to join them in destroying the world. Upon choosing not to, Chara removes control from the player and destroys it themselves. Upon choosing to destroy it, Chara calls the player a great partner and they will be together forever. The end games with Chara and the player destroying the world and possibly move to another to destroy.

The game will close once that happens, if the player opens it again they will only encounter a black screen what happens to be the Undetale world completely destroyed. If the player waits, Chara will appear and offers them to return the world in exchange for their soul, if so then the world will be restored but at the same time some elements on the True Pacifist Route will be altered such as Chara replacing Frisk on their bed and staring to the player with red eyes if they chose to stay with Toriel and the photograph with the main monsters being altered with their faces having a red X and Chara replacing Frisk if the player chose to "have places to go". All this means Chara has taken full control over Frisk and they proceeded to kill everyone there again just as they did before in a Genocide run, this is by far the worst ending the player can get, as Chara wins regardless.

However, the player can steal their soul back and restore the game to their will by editing the game files and removing Chara from control.

Neutral and True Pacifist Routes
At any time, should you choose to spare any monster (save Sans, who cannot forgive you), Chara's control will be permanently destroyed.

Trivia

 * Chara notes that they were confused at first upon awakening, and the player taught them just how essential power is for success.
 * This arguably means that the true main antagonist ( or at least the Bigger Bad) of the Genocide route is the player themselves, as they are responsible for Chara's evil and exist outside of their control.
 * Even when Chara removes control from the player by deleting the world, the player can regain control by editing the game files to their will.
 * There are some secrets and easter eggs found during a Genocide run which foreshadows Chara's return throughout the game:
 * At the rain on Waterfall, if one looks closely the puddles of water they will notice a creepy figure reflecting which looks similar to Frisk except for their clothes and color palette.
 * In the Lab, if the protagonist sees the monitor in the area, it won't follow their moves as it did in a Neutral or Pacifist run, instead it will quietly keep staring to the player.
 * Chara is similar to Emperor Palpatine as:
 * Both drove former heroes into the dark side (Asriel Dreemurr and Anakin Skywalker)
 * Both serve as Bigger Bads while at first they were never considered the true main antagonists until revealing their true intentions.
 * Both died for a while but then they sought a way to return from death.
 * Both hold a hatred towards a group or species, Chara to humans and Palpatine to the Jedi.
 * It is implied that they are the narrator in a Neutral or Pacifist route as well, making them a benevolent character who helps guide Frisk through the underground, implying that the True Pacifist route is the road to their redemption for what they wanted to do to humans.
 * If you decide to laugh at Snowdrake's mother Chara is the one that starts laughing while Frisk doesn't and if the player tries to laugh again they say that it isn't funny, implying laughing in stressful situations was a coping mechanism while they were alive and before being corrupted by the player.