Betê Noire

Bête Noire was created in the aftermath of a conflict between Agate, Copper, and Amber Lightvale. Before she existed, her soul resided in Agate Lightvale.

When Agate and Copper began to argue over the barrier, their tempers flared, and they fought, while Amber watched. At the end of the battle, Agate lost, humiliated; thus, she lost her trait and fled.

While Agate was gone, she sought a trait that could defeat Determination.

When Agate returned to the kingdom, Copper and Amber gave her a warm welcome due to her long disappearance. However, what they didn't know was that she had inverted her soul, so after some time, she fought her brother against his own will. Amber tried to escape during the fight, but her sister blocked her from escaping by using one of her magical blasts and murdered her. This caused Copper to lose his trait and sit there, frozen in fear while his sister was killed in front of him. She then stabbed her brother, killing him. Agate then realized that her soul was damaged, and she was slowly dying after she inverted her trait. This damage was because her soul couldn't adapt to the sudden change of it being inverted. Knowing that her time was running short, she used her soul and Amber's body for a spell that would create a powerful creature that would never let humans and monsters live in peace, the Bête Noire.[2][5]Bête Noire was a construct of a spell and a physical incarnation of the SOUL Trait Fear, created by Agate Lightvale solely to ensure humans and monsters would never live in peace. A thousand years after her creation, monsters were freed from the Underground and lived in harmony with humans, so Bête, under the alias Betty, met Frisk and posed as an ally. During her temporary acquaintance with Jessica Grey, they both were responsible for framing Papyrus and the murder of Sans. After Bête's true nature was revealed, her actions started expanding into a war, where she ruthlessly slaughtered humans and monsters alike to obtain their SOULs. Bête began losing track of her mind when she was enhanced with HATE, but nonetheless, Bête continued her murderous attacks on Determino, fighting her enemies physically and emotionally. However, in her final fight against W. D. Gaster and his two children, Gaster, out of pure rage for Sans's second death, brutally tore Bête apart and struck her with endless blows, finally destroying Bête once and for all, but it resulted in the release of HATE, who was defeated by Chara and Asriel.