Twelve

Twelve was a psychic girl from Patrickia who had the ability to open and close openings to and from the Oopside-Done. She also had telekinesis, with a light-yellow aura, which she used to decapitate the Demogorgon with (who didn't actually die). She nearly got crushed by Mind Guy's giant arm, and instead died when Lucas tried to use fireworks to kill said demon.

Twelve is the sister of Sans and Greg, and lived in their house for a bit before leaving with a gateway she made. She obviously knew of Mind Guy's evil ways when he started to invade, and attempted to save the town of Bruhkins.

Twelve was a member of the Jackson family.