Cosmo

"Halloween!"

Cosmo (コスモ) is the Cosmos Fiend (の), and was one of Quanxi's girlfriends and represents the fear of omniscience and gaining all knowledge of everything in the universe.

Appearance
Cosmo possesses a corpse with 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 is in constant 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 telepathically send a person’s mind into her own mind that seems to be a dimensional space that resembles 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.  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.
 * Omniscience: Cosmo has all the knowledge in the universe inside her head. This essentially gives 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". She doesn’t seem to be omniscient when she isn’t using her ability as in many circumstances she seems to not know or expect many things and in contrast to her neutral inner personality, who accepts the fate of hunting or be hunted to be natural, seemed more concerned with death and freaks out in dangerous situations. 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

 * Cosmo's rankings in the Popularity Polls are as follows:
 * Cosmo ranked 34th in the First Popularity Poll with 343 votes.
 * Cosmo ranked 20th in the Second Popularity Poll with 3,527 votes.
 * Cosmos's name wasn't revealed before the Character Popularity Poll, in which she is referred to as Zombie (ゾンビ) from Quanxi's group.
 * Given the omniscient nature of her powers, Cosmo should be able to know and remember everything even after reincarnation. But it is unknown if her personality will also reset, as each incarnation of the same Devil is a different person.
 * It is unknown how or why the Cosmos Devil became a fiend, as her true personality reveals that she does not care about death, accepting it as natural order that cannot be changed and was shown to be quite indifferent towards it, and unlike other devils she would not have to worry about loosing her memories as knowing everything is a natural ability of her’s.
 * Aside from the word “cosmo” another translation for the Japanese “Uchu” part of Cosmo’s Devil/Fiend name would also be “universe” and “space.” Although her fear is more on the specific fear of gaining infinite knowledge of space rather than the more generalized aspects and concepts of space such as the reverse fear of fearing space because one doesn’t know much about it.
 * While Cosmo’s fear seems like it would fall under or be connected too “the fear of the unknown” it is actually its direct and polar opposite given how it’s “the fear of knowing everything” which is befitting to the theme of “ignorance is bliss” in the arc she appears in.
 * Her power is very similar to the Jujutsu Kaisen character Gojo Satoru’s Domain Expansion ability called Infinite/Unlimited Void. While Cosmo’s power is more simplified and less powerful, Gojo’s ability is more powerful, massive in scale, easier to activate and is more complex as it makes the target know, sense, see and feel everything and nothing at the same time and puts them into catatonic and incapacitated state. However, according to the mangaka but not explicitly stated in the story, it is actually explained that the ability is just that the target is having their thoughts repeated infinite times causing the illusion of having infinite knowledge and even then the damaging effects are still the same.