Chara

Chara is the named character in Undertale. They were a kind-hearted child, who wanted to free the monsters with Asriel, by having Asriel collect seven souls (they were one of the seven!) Asriel refused, and they died.

When they were reawakened, it was without a SOUL. This, Asriel's death, and outside corruption transformed Chara from a good kid to an evil monster who wanted to kill and destroy everything.

It didn't help that Chara was materialized into World Prime by N. ??? killing a huge number of bunnies. They joined the Dark Side, as they wanted to kill off the Light Side.

Eventually, however, the Care Bears brought them over, and with their Care Bear Stare, they were able to remove the corruption from within, restoring Chara to their former, good self.

Trivia

 * According to the canon of the ETQAY Saga, Chara was the narrator in Undertale-This would of course be before they were corrupted. Their corruption began when Frisk attempted a Genocide Run and gained LOVE, corrupting their past as well.

It’s unknown why they vanished in the first place, however, nor how they reawakened when N. ??? gained a bunch of LOVE.