Eve

Eve was a mysterious and powerful being known by the title of Mother of All. She was in the depths of Purgatory until she was released by dragons and arrived on Earth using a young virgin girl as a vessel. Like the Leviathans, to whom she was related, Eve was part of existence before angels, which gave her an advantage over them, being able to inhibit them from using their holy powers.

Personality
Like Death, Eve was normally content to uphold the natural order in which monsters and humans killed each other in turn; however, when someone began capturing, torturing, and killing her children in droves, she decided to turn every human into a monster to deprive him of the power of any souls. She exhibited few emotions beyond serenity, but her decision to pose as a waitress merely to surprise Shiro and Tsubaki suggests that she had a sense of humor. She seemed to be upfront and honest when speaking with Soren and Tsubaki. She also had a mocking side to her sense of humor as she took their mother's form to taunt them.

Eve was largely apathetic towards the human race as a whole, but occasionally showed disdain or cruel amusement towards them. She began compelling even her benevolent children to kill or turn as many as possible against their will once she arrived on Earth, presumably to build her army. She went about trapping a bar filled with people, infecting some, and allowing the others to be killed while she watched on and casually sipped at a drink.

When explaining what she had done to someone she had turned, Eve simply called her infection "a gift". She viewed herself as a loving mother with regards to all monsters descended from her. While viewing herself as a loving and caring mother to her children, she is not above killing any of them if they lose a threat to her other children as she created swords specifically designed for killing the very powerful dragons, should they become a threat to any of the other monster races she has made. She also seems to look down on God, whom she claimed to know, for having abandoned his children, i.e. humankind, something she claims she would never do.

Early Life
Eve predated angels (except Archangels), which would dictate that she also existed before the creation of Hell, humans, and Demons. Her origins seem to overlap with that of the Leviathans, a primordial, proto-monster species which also predated most other creations of God. Soren acknowledged that the Monsters Eve sired do come from the Leviathans, though "barely", and confirmed that he knew Eve. At some point of her long existence, Eve created the Alphas, which were the first monsters of their allotted species. Before re-emerging in the 21st century, Eve had last been on Earth around ten thousand years ago.

Powers and Abilities
Eve was very old and powerful, stronger than any monster. Her power compared to that of the Leviathans is thoroughly unknown. However, being related to them, she shared some abilities with the Leviathans, notably a resistance to angelic powers and shapeshifting.

Eve was also one of the very few entities capable of creating new beings and one of the only beings with the rare ability of permanently altering souls to create monsters otherwise known as Fallen Beasts, a trait she passed on to all of her children. Eve also referred to the souls in Purgatory as her 'supply', implying she could draw on souls for power, not unlike angels and demons.
 * Immortality - Eve was immortal and could not die of disease or old age. She referred to herself as being older than even the angels.
 * Invulnerability - Conventional weapons and most mystic ones did not have any effect on Eve. The only known thing that could kill her was Phoenix ash, either by ingestion, being injected with it, or being shot with shotgun shells filled with the ashes.
 * Flight - Once she took her vessel, she rose from Purgatory.
 * Possession - In order to manifest on Earth, Eve required the use a vessel, and that vessel had to be a virgin girl.
 * Biokinesis - She was able to create new species by mixing the other monster species. Eve, upon possessing her vessel, was also able to greatly alter her vessel's physical functions. The most notable traits were when her womb was altered to reproduce several hundreds of monster eggs. The other alteration was when even dead, monsters were linked to her cerebral cortex & could feel everything she felt, even if it was a small tingle.
 * Conversion - She could turn humans into whatever kind of monster she desired, through physical contact, a bite, a kiss, or even a gentle touch. She apparently needed to perfect this ability when attempting to produce a new kind of monster, seeing as how a number of her creations died prematurely before she eventually succeeded in creating "the perfect monster." She dismissed these failures as "beta tests."
 * Monster Birthing - As the Mother of All monsters, she can also give birth to them.
 * Regeneration - When she rose from Purgatory, Eve's arms and legs were covered in severe burns, however she later reappeared completely restored.
 * Super Strength - Her strength was far superior to humans, many earthly supernatural creatures and even regular angels and seraphim. Humans were no threat to her in terms of bodily force. This was shown when she ripped a lock and handles from a door.
 * Super Speed - She could move at undetectable speeds for a human being, allowing her to reappear in different locations at will. She displayed this by instantly running behind Tsubaki while having been talking to her face to face less than a second before and when she appeared out of nowhere behind a truck driver.
 * Shapeshifting - At one point, she modified her vessel's appearance to look and sound like Renee. Her shapeshifting was advanced to a degree seen only on few other creatures—the Alpha Shapeshifter, the Leviathans, and the Archangels.
 * Telepathy - She was psychically linked to all monsters, allowing her to know what they do and speak to them through their minds and could communicate with her children even while imprisoned in Purgatory. According to her, she was aware of the hunters' arrival and intent to kill her before they met. She could also read a human's mind, as she was capable of shapeshifting into Soren and Tsubaki's mother despite never having seen her, and knew that she died to protect them.
 * Compulsion - Using her mental link to her children, Eve could control the various monster lines and instill bloodlust in them, even over great distances including through dimensions as well, as seen when she compelled her many children even the more benevolent ones to attack and turn as many humans as they could. She was residing in Purgatory at the time.
 * Super Senses - She possessed heightened senses.
 * Angelic Power Negation - Due to her advanced age and knowledge, Eve was able to block angels from accessing their powers by 'unplugging' them. Her overall knowledge over angels was extensive. She shares this ability with the Leviathans.
 * Supernatural Concealment - The protagonists had to rely on the willing testimony of a monster ally to know her whereabouts. Like Angelic Power Negation, this is an ability Eve shares with the Leviathans.

Weaknesses
Eve is quite difficult to destroy; only a few weapons and forces are able to kill her.
 * Phoenix ash - Ashes of a phoenix can kill Eve.

Trivia

 * Her name references the Biblical Eve, who is regarded as the mother of the human race in Abrahamic religions.
 * Her character borrows the title "The Mother of All Monsters" from Echidna of Greek mythology.
 * In addition to the Greek Echidna, as mentioned above, Eve also strongly resembles the Babylonian Tiamat, though she claims to have visited Earth more than 6,000 years before the founding of the first Babylonian dynasties (1894 B. C. E.), and the Semitic Lilith, not to be confused with the character of the same name.
 * Soren said he knew Eve, referring to her as a "whore".
 * Even after her death, her body still produced new monster eggs.
 * Despite taking a vessel, her true visage or at least a portion of it could be revealed on a camera recording.