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 agrees to destroy the world. "No...? Hmm...How curious. You must have misunderstood. SINCE WHEN WERE YOU THE ONE IN CONTROL?"

- Chara if the player turns away the offer to destroy the world.

Chara (last name Dreemurr upon being adopted by Asgore and Toriel) also known as the Fallen Human or First Human, is a posthumous character and, ultimately, the true main antagonist of the 2015 videogame, Undertale, being indirectly responsible for the events of the series and the creation of Flowey. However, they also serve as the main villain of the Genocide Route if the player decides to kill every monster in existence. They are also heavily implied to be the narrator that Frisk talks to, thus only making them the villain in the Genocide Route, and a protagonist and remorseful character in True Pacifist and Neutral.

Chara, at full LOVE, is the only being in Undertale capable of directly challenging the player's influence over the world. However, they are still powerless against the player's indirect attempts (editing the game files) at controlling the world.

Backstory
For unknown reasons, Chara seemed to despise humanity and its actions, despite 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 young Boss Monster 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 goaded Asriel into a plan. Upon Asgore being poisoned from buttercups, Asriel was terrified, while Chara was said to laugh it off, either in amusement or to cope with the accident. Chara devised their gambit by desiring to use a SOUL to destroy the barrier between the Underground and the surface. They fatally poisoned themselves by ingesting the buttercups, and after the death of their body, Asriel absorbed Chara's soul. Transforming him into a beast, Asriel and Chara ascended to the Surface, intending to destroy all humans and undeterred by both of them 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 (as monsters turn to dust upon dying) sprinkled around the burial.

Due to the experiments of Alphys, Asriel's remains and memories were placed into a Golden 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 SOUL and consciousness, however, remained dead.

Genocide Route
Chara is the true main antagonist of the Genocide Route. As Frisk kills more and more monsters, Chara begins to assume 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.

Upon arrival at Asgore's palace, Sans confronts Frisk/Chara, surprisingly and efficiently proving to be the most powerful character in the game. Despite having the weakest of defenses - his DEF stat being a mere one - he is the only monster who dodges the player's hits, which makes the entire battle a struggle to land a single, fatal blow. The last hit is delivered by the player and misses, before Chara takes on control and manages to strike Sans by surprise. He then reminds them that he warned them of the consequences before heading off to Grillby's and seemingly dying and dissolving off-screen. Chara (and Frisk) proceed to attack Asgore even when he kindly spoke to them - because he failed to recognize Frisk's body as that of a human. Asgore is then killed by a horrified Flowey, who as enthusiastic about the player's choice to destroy the Underground's entire population, but who knows he is going to be killed as well. Flowey claims that he had never betrayed Chara - despite warning Asgore of Frisk's arrival - and begs them to not kill him as he contorts his face to mimic Asriel. Chara gives back control to the player at this point, being hesitant to kill Flowey - despite having murdered their adoptive father themselves - and giving the player one last chance to back out of the Genocide Route by closing the game.

If the player continues, Chara completely slashes him out of existence, leaving nothing behind. 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. Regardless of the player's decision (though if the player refuses, then Chara will betray them and override their control), Chara destroys the world and closes the game in the process.

If the player reopens the game, they will only encounter a void where the Underground once was. After ten minutes, Chara will appear and offer them to return the world in exchange for their soul. To fully restore the world, the player has to accept; otherwise, Chara will leave until the player restarts the game and waits ten more minutes again.

If the world is restored and the player completes a True Pacifist Route, it's endings are tainted. Regardless of which option Frisk chooses at the end, Chara possesses Frisk and presumably murders every one of their friends. If Frisk chooses to stay with Toriel, Frisk wakes up with Chara's features and emits Flowey's signature laugh. If Frisk claims to have places to go, Chara replaces Frisk in the picture in the end and seemingly crosses everyone else's faces.

If the player completes the Genocide Route a second time, Chara will greet the player again. Chara describes The Anomaly as "wracked with a perverted sentimentality", that not even they could understand and suggests trying the True Pacifist Route. Chara will then make the same offer to destroy the world as they did before, with the same results of the player's choice in both routes.

Neutral and True Pacifist Routes
At any time, should the player fail to meet an area's requirements to continue the Genocide Route (save Sans, who cannot forgive you), Chara's control will be permanently destroyed.

There do exist theories that Chara is also the narrator in the True Pacifist and Neutral Routes, as indicated by differences in perspective between the Genocide Route and all others. Chara could also be the true protagonist of the game, as they may represent the player up until the Genocide Route's conclusion. It's suspected that in the True Pacifist Route, Chara willingly guides Frisk to make amends for their misdeeds and is the one in control in every battle.

Of note is the fact that Flowey, and later Asriel, when he regains his original body at the end of the True Pacifist Route, mistakes Frisk for Chara and seeks in them the return of his lost sibling. When he realizes his mistake, the player learns that the name Chara - or any name they chose at the creation of their save file - isn't the protagonist's, but the first human's. This realization doesn't occur in the Neutral or Genocide Routes.

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 the Genocide Route, 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 the Neutral or True Pacifist Route. 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.
 * Chara shares some similarities with Yuuki Terumi from the BlazBlue series:
 * Both are responsible, directly or indirectly, for everything that happens within their games.
 * Both use knives to kill or torture enemies.
 * Both are among the most powerful villains in their respective games.
 * They constantly taunt the player/protagonist by throwing psychological insults.
 * Both have hosts with their eyes shut (Frisk, Hazama).
 * They did everything in their games just for fun in Terumi's case.
 * It is implied that they are the narrator in the Neutral or True Pacifist Route as well, making them a benevolent character who helps guide Frisk through the Underground, implying that the True Pacifist Route is the path to their redemption for what they wanted to do to humans. This, however, is not confirmed.
 * 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.
 * Chara is a real world name in multiple languages. In Greek, Chara means joy. In Old French, Chara is a shortening of Charolette which means little and womanly. In Spanish, Chara is the word for face. All of these names are female names; this explains why Chara is referred to as female in most fanworks. However, it can mean dude or buddy in various languages, so there is room for Chara being identified as male.
 * Chara is often referred to as the main villain of the Genocide Route. However, it is heavily implied that Chara only becomes as evil as they are due to the player's interference, making the player (The Anomaly) the true main antagonist, with Chara only being a result of the player's own actions.
 * Because Chara will bear the name the player inputs for them, they may also be intended to be the true avatar of the player.