Chara is the 1st Human that falls in the Underground. Asgore and Toriel adopted her as the Sister from Asriel . Later, Asriel and Chara founded out that when Asriel is using Charas soul DETERMINATION, He can break the Barrier and bring all Monsters to the Surface. So Cahara killed herself, Asriel took the Soul and broke the Barrier. But the Humans thougt HE killed Chara . So they started to Attack Asriel and build the Magical Barrier again. Asgore and Toriel, that lost 2 Kids in one day, grab chara under a flowerfield . When the Human Frisk falls, She wake up as a Spirit and follows Frisk in her Adventure, remembering that the Humans killed her Brother.
When the Player is doing Pacific, Chara is learning that there is ever a way to continue without Brutality.
When the Player is doing Genocide, Chara loose everything (Toriel , Asgore , Flowey...) She gets crazy andd take over the Player, kills Frisk and take the Soul away. Then is the Player never be able more to do Pacific.