Cosmo

Cosmo is the Cosmos Fiend (コスモスの魔人, Kosumosu no majin) and was one of Quanxi's harem members.

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

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".

Supernatural Abilities
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.

Contract Creation: Cosmo can make contracts with humans just like any other Devil.

Revival: As a Devil, when Cosmo gets killed she is revived in Hell making her effectively immortal.

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.