Chara
Chara is the player's character in the game, with their name changing as the player wishes. They have played a major role in the events preceding the game, and are the true antagonist depending on which route the player takes.
Chara makes a prominent appearance should the player take the Genocide route. As they player goes to leave the Ruins after killing everyone, Flowey will appear say that your actually Chara and that the two of you are still inseparable after all these years. As the player kills more of the monsters, they'll start to see red colored text when they go to SAVE their progress indicating how many monsters are still alive. And when the player reaches New Home and interacts with the everything on the way to the castle, much of the dialog will be colored red and be spoken in the first-person perspective. This is because as a result of all the murders the player has committed, Chara began to gain some control over the player's character and now their own personality is starting to show through. Based on the dialog, Chara shows themself to be rather pessimistic and focused on the task of killing, with little regard for "comforting" things such as Asgore's Santa outfit. As the player gets closer to the castle, Flowey will ponder about Chara's power and how they managed to come back. In regards to Chara's power, Flowey has noticed that after failing to kill Chara at the beginning he couldn't reload his SAVE files to try again, stating that Chara's Determination is stronger than his own. As for how Chara managed to come back, Flowey surmises that Toriel must have taken Chara's remains when she left and gave them a proper burial at the ruins, and wonder's if his original cries helped to awaken Chara. Thinking that Asgore might show Chara the six human Souls, Flowey suggests that maybe they should carry out their "original plan" of destroying the Barrier and going to the surface, saying that it will be fun to be with someone who actually understands him. However, after making the statement of "Creatures like us wouldn't hesitate to KILL each other if we got in each other's way", Flowey will start shaking with fear and say "no hard feelings" about when he attacked back in the Ruins. Based on Flowey's statements about how Chara is approaching him, it can be interpreted that Chara does indeed hold a grudge for the attack and is considering on acting upon it. Chara's Determination to see their goal through can be seen in the boss fight against Sans; when Sans breaks a "game rule" by doing nothing to prevent Chara from having another turn, Chara will in turn break the rules by pushing the action box until it's over the "FIGHT" option and then attacking twice in one "turn" to kill Sans. After defeating Sans, Chara will find Asgore in the garden, who will not recognize them and even question what kind of monster they are. Just as Asgore is about to suggest talking about it over some tea, Chara will initiate a battle and attack Asgore without the player's influence. After Flowey delivers the final blow to Asgore in the hopes of being spared, Chara will brutally kill him, again, without the player's influence. Chara will then introduce themself and thank the player for awakening them from death, specifying that they weren't using their "SOUL" and "Determination" throughout the game but the player's. Further more, when the players HP, ATK, DEF, GOLD, EXP and LV increased, that was really indicating just how much closer Chara was to being able to revive. Saying that there's nothing left now, Chara will tell the player that it's now time to erase this world and "move on to the next." Should the player refuse, Chara will make the remark of "Since when where you the one in control?" before attacking the screen and crashing the game.
Should the player open the Undertale app after completing the Genocide route, they will be presented with nothing but a black screen accompanied by the sound of howling wind. Should the player try to play the game again, Chara will speak up saying that it's "interesting" how you want to go back to the world you destroyed. Asking if you think you are above consequences, Chara will offer a compromise; they will restore the world to play in again, in exchange for the player's Soul. Doing this and then playing for the Pacifist route will give the Soulless Pacifist ending, wherein Chara will reveal themself after Toriel leaves a piece of butterscotch pie in the bedroom, reminding the player of what they've done in the first place.
If the player goes for the True Pacifist route on their very first playthrough of Undertale, Chara will be mentioned but will never actually appear in the story. The player will hear the most about Chara when they arrive at New Home and start working their way to the Castle, in the form of monsters appearing to tell the story of the first human to come to the Underground. Long ago, Chara fell into the Underground through the caves above the Ruins. When they called out for help, Asriel heard them and brought them back to New Home. Overtime, Chara became like a sibling to Asriel, and was even treated as such by Toriel and Asgore. Then one day, Chara became fatally ill and had only one last request; to see the flowers that grew in their home village. When Chara died, Asriel absorbed their Soul and passed through the Barrier carrying their body back to the village. Once there, the humans thought that Asriel had killed Chara and attacked him. Chara's death, and Asriel's death that followed soon after, contributed to Asgore's declaration of war against the humans and his new policy of killing every human that comes into the Underground. When the player goes into the True Lab, they'll come to a room full of VHS tapes including some home videos from when Chara was living with the Dreemurr family. Some are videos Asriel took of his interactions with Chara, which are mostly of lighthearted fun the two had but one features Chara using name-calling and accusations to convince Asriel to help out with a plan to make them stronger. The final tape is of when Chara fell ill, most likely as a result of consuming some flowers as suggested by previous videos. Toriel and Asgore can be heard begging Chara to stay alive, while Asriel will be heard saying that he doesn't like the plan anymore but take that back saying that the two of them will find six Souls to free everyone. Later on, when Asriel reappears after absorbing the human Souls and all the monsters' Souls, Asriel will initially refer to the player character as Chara but then say "Your not actually 'Chara'" as he remembers that they died long ago. Once aware of Frisk being the player character's true identity, Asriel will purge what's left of Chara from them while he restores the world and destroys the Barrier. Should the player return to the beginning area before leaving for the surface, they can find Asriel at the bed of flowers where the player can learn more about Chara. According to Asriel, Chara climbed the mountain connected to the Underground because they hated humanity, and though they never said why Chara held strongly to that hatred even when living among the monsters. With the exception of clothing choices, Asriel will point out just how different Frisk is from Chara and that he's surprised that he tried to treat Frisk as the same person. With this, Asriel will realize that Chara probably wasn't the best influence on his life, especially recalling when he absorbed their Soul. After absorbing Chara's Soul, control over Asriel's body was split between the two individuals as it was actually Chara who picked up their own corpse, and when the humans attacked Chara tried to fight back. It was partially because of Chara's influence that Asriel came to develop the view of "kill or be killed" as Flowey, but now, after all that's happened, Asriel sees that he did the right thing by holding Chara back. Chara will not appear unless the player chooses to take the Genocide route.