Cosmo

Cosmo (コスモ) is the Cosmos Fiend (の), and was one of Quanxi's girlfriends.

Appearance
Cosmo has spiky pink shoulder-length hair fashioned into pigtails with red hair bands. She also has hearts for pupils, colored either orange-red or yellow-red, and her right eye usually hangs out of its socket. It is inconsistent how much her right eye sticks out but it is probably retractable as in one instance it seems as if she can move the eye on its own. The upper right side of her head is gone, exposing her brain, part of which trails down past her face. The trailing part of her brain is tied into a bow at the end. She wears a long sleeved cropped orange sweater, light blue high waisted shorts, and pink sneakers.

Personality
On the surface, Cosmo doesn't exhibit many emotions aside from her usual smiling, 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 Santa Claus, the two had a conversation in Cosmo's mind, revealing her true inner personality. Here, she is capable of talking normally, conversing with Santa Claus about what's happening. She is straightforward and formal in her manner of speaking, seeming far less excitable than her outward personality. Cosmo was not particularly upset about her girlfriend's murders, explaining that she has no emotions strong enough to drive her to be upset about "the course of nature". Her true personality therefore seems more logical, detached from others, and mostly devoid of emotion.

Her surface personality might be the result of her powers affecting her, causing her to only say "Halloween." However, unlike those she inflicts her power upon, Cosmo is still capable of understanding the speech of others and seems to be capable of thinking, as she is able to take orders and carry them out just fine.

International Assassins arc
Cosmo is shown as one of Quanxi's girlfriends, alongside Pingtsi, Long, and Tsugihagi. Like the other fiends, she is loyal to Quanxi and can be seen following her around during the arc. She never says anything other than "Halloween." After Santa Claus kills Quanxi's other girlfriends besides Tsugihagi and threatens to make Quanxi one of her dolls, Quanxi tells Cosmo to give Santa Claus a taste of an all-out Halloween. Cosmo obliges, and brings Santa Claus into her mind telepathically.

Santa Claus observes that Cosmo intends to launch a psychological attack on her, but alledges that it would be ineffective. Cosmo then responds in normal speech to introduce herself as the Cosmos fiend, surprising Santa Claus. Cosmo explains that while she has no personal anger towards Santa Claus, the attack that will be launched on Santa Claus' mind is simply the natural order. She further tells Santa Claus that she is about to gain total knowledge of everything in the universe, and afterwards, she will be incapable of thinking about anything other than Halloween until she dies. This is exactly what happens, and Santa Claus and all of her dolls are left semi-catatonic, only saying and thinking of Halloween. Despite her help in defeating Santa Claus, Cosmo is still beheaded by Makima along with Quanxi and Tsugihagi.

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 infinite knowledge of the cosmos and outer space, the more fear people have towards gaining total understanding of everything in the universe and similar concepts, the more powerful Cosmo will become.

Devil Powers

 * Total Understanding: Cosmo can send a person’s mind into a different dimension that seems to resemble a gigantic library, she would then force the target to understand everything in the universe. This renders them into a semi-catatonic 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: Cosmo has all the knowledge in the universe inside her head. This essentially makes her a possibility to be omniscient, however, due to the fact that Cosmo herself appears to be affected by her power likely because of her weakened state due to being a fiend, she is unable to utilize or access this knowledge in any meaningful way, being only able to think and say the word "Halloween". Cosmo may only be able to think completely normally when she is inside her own dimension talking to someone else that she sent there, but she does have a higher degree of functionality than her victims even outwardly, as she is not rendered semi-catatonic by her power.

Trivia

 * Her name wasn't revealed before the Character Popularity Poll in which she is referred as Zombie (ゾンビ) from Quanxi's group.
 * Cosmo’s design, as well as constant shouting of the word “Halloween!” gives the false impression that she is something zombie related or something similar. In actuality, her design is meant to represent how her brain is exposed due to such vast knowledge.
 * It can be a bit of a trope in media, especially sci-fi, for many “smart” characters to have an exposed brain to reflect on their knowledge or intelligence.
 * The additional popped out eye is likely because her brain is so huge (either symbolically or literally) that it even causes one of her eyes to pop out. Her popped out eye seems to be retractable so another reason is that this could have less of a deeper meaning and is just adding onto the zombie facade.
 * It is heavily implied that the target needs to copy Cosmo saying “Halloween” after she says it, and possibly for there to be some kind of direct focus, in order to work.