Cosmo

Cosmo (コスモ) is the Cosmos Fiend (宇宙の魔人), and was one of Quanxi's girlfriends.

Appearance
Cosmo has short shoulder-length hair that's fashioned into pigtails. She also has hearts for pupils and the right part of her head is missing, making her brain visible with some of it hanging in front of her. Her right eye is also hanging from a tendril.

Personality
On the surface, Cosmo doesn't exhibit many emotions aside from her usual smiling, seemingly happy-go-lucky attitude. She doesn't speak except for repeating the word "Halloween" and, like the rest of Quanxi's fiends, follows Quanxi around and seems to be in love with her and under her command.

When Cosmo uses her powers on Master, another version of herself appears during their conversation, which probably happened in Master's or her mind, hinting that that's her true personality. While she retains her happy demeanor, she is shown to be calmer and able to talk normally, conversing with Master about what's happening.

Her surface personality might be the result of her powers affecting herself, causing her to repeat and not be able to think about anything but "Halloween."

Abilities
As a fiend, Cosmo can't make contracts. However, she still has all the other standard devil abilities. As the fiend that embodies the fear of the cosmos, the more fear people have towards outer space and similar concepts, the more powerful Cosmo will become.

Devil Powers

 * Total Understanding: Cosmo can force her target to understand everything in the universe. This renders them in a semi-catonic state in which they are "only able to think of Halloween until they die." Once this occurs, the victim will be unable to think or say anything other than the phrase "Halloween" until their death.
 * Omniscience: Due to the fact that Cosmo herself appears to be affected by her power, Cosmo has all the knowledge in the universe inside her head. This essentially makes her omniscient, however she is unable to utilize or access this knowledge in any meaningful way, being only able to think and say the word "Halloween". Cosmo only appears to be able to think normally when she is using her powers on someone else.

Trivia

 * Her name wasn't revealed before the Character Popularity Poll in which she is referred as Zombie (ゾンビ, Zonbi) from Quanxi's group.