Gaster vs Bete Noire (Battle)

"You're the first of your kind. I'll make sure you're the Last."

- Gaster talking to Bete Noire

Gaster teleported Asgore, Asriel and Frisk to the school in order to protect the children there.

While they were leading the battle on the school, Gaster decided to track down Bete with the help of Alphys and Jessica Grey.

Somehow, with Jessica's device, they found Bete Noire's location and decided to go to the CORE.

Battle
Soon after that, they figured out Bete's location, Gaster trapped her inside of the CORE's expansion and decided to fight her. Gaster had been extremely close to beating Betty however due to her Rhadhophobia attack she was able to use his own attacks against him, defeating him.

At the end of the battle, Bete speared his soul without killing him. But Jessica and Alphys interposed and neutralized Bete using the AMR.

Alphys continued with Gaster's plan (At the beginning, Alphys had to activate the Nullifier room in order to kill them) and decided to close herself with Bete in the nullifier room with a shield active.

Finally, Alphys ended up with a spear in her chest and the nullifier failed to exterminate Bete and Akumu. Undyne came to rescue her, only to accidentally kill her.