Chara

Chara is SunsetBlvd's favorite Undertale Character.

Personality
Chara is mischevious, and somewhat evil. Most people say to not trust her, because you can only find her in Undertale by doing the genocide route, in which you must kill every monster in the game. Also, by doing genocide route, Chara can actually possess you. Two examples of this are when Sans tells you to hide behind the conveniently shaped lamp, and you refuse, and when Papyrus says to not step closer but you do anyway without a choice.

Story
Chara was the first human to fall into the underground. Just like Frisk, she had the soul of determination. After she fell, Asriel found her. The two became friends, and Asriel took Chara to her parents, Toriel and Asgore, who was the king of the monsters.

Chara was a great addition to the family. But one day, Chara felt ill. When she was dying, she told Asriel to absorb her soul, which he did, turning him into a more powerful looking monster. She then told Asriel to let her see the golden flowers in her village when she died. Asriel did as instructed, crossing the barrier with Chara, almost dead, in his hands.

When he got to the village with Chara, the villagers saw him. They thought Asriel was the one who had killed Chara. In a state of anger and fear, they all attacked Asriel, weakening him with each hit. When he got back to the underground, he put Chara's dead body on the ground, and seconds later, Asriel died and turned into dust, right in front of Toriel and Asgore's eyes.

When they witnessed this, Asgore stated that once again, the humans have taken everything. If any human ever fell into the underground ever again, he would kill them all to absorb their souls and destroy the barrier. When Toriel heard this, she was outraged. She told Asgore that she would find the humans and keep them safe from him. The two never saw eachother again until Frisk arrived.

This video will explain better with pictures what happened: