Chara

Chara is a former resident of the Underground.

Chara's only motivations are murder, and Chara does this out of boredom. Chara has the power to SAVE and RESET, which helps Chara avoid death and wreak havoc if something doesn't go Chara's way.

UndertaleFanboy11037
She died quite a while ago to kill the villagers from her mom, but due to DETERMINATION to eradicate the world, she is still around.

She is most commonly seen wearing a hood, hiding her identity.

HideFromMONSTER
Chara is seen in her human form at the end of the Genocide/No Mercy run,but has a Demon form,too. Chara's demon form looks like a tall,demonic figure with long arms. Chara can turn her limbs into a knife,having a knife for a hand is one example.

UrbanPie
"Chara is an unstoppable best" this statement is untrue, Chara can be killed with ease, her determination is why she is still living. If you could sap her determination, she would render useless.