Queen Nehellenia is a beautiful woman, though confirmed to be several centuries old throughout the duration of her arc. She has long and wavy, floor-length glossy black hair with two bunched up buns at the sides of her head with separate strands of hair streaming down from them. She has fringes across her forehead that points inwards towards her nose, and she wears a small head jewel, which is silver and has a circle with obsidian in the middle of it, and three crescent moons drooping down from it, one gold and two silver. Nehellenia's most noticeable facial features consist of large ice-blue eyes, crimson lips, and pointy ears, in which she has earrings that are made of three moons each in a similar design to her headdress. She also wears a large, golden choker in small strips of plating encircling her graceful neck with black pearls rimming the edges. Her outfit consists of a dress, with large puffed sleeves that end in gold bracelets, a deep V-neck and a long, floor-length flowy skirt that is initailly black but covered over with a slitted ivory sheet of silk.
Queen Nehellenia was, in essence, the dark mirror image of Queen Serenity, the self-styled "queen" of the dark center of the moon who had existed since the Silver Millennium. She first made herself known at the celebration of Princess Serenity's birth, at which point she confronted Queen Serenity, who used the Moon Stick to seal Nehellenia inside a dark mirror forever. In retaliation, Nehellenia cast a curse that the Moon Kingdom would fall and the princess would never inherit the throne. Due to being sealed inside her mirror world, Nehellenia was able to live on even after the destruction of the kingdom.
Thousands of years later, a solar eclipse allowed her to breach the seal. At this point, she found Sailor Chibi Moon's four future guardians, the Sailor Quartet. She awakened them prematurely and brainwashed them into becoming her minions, the Amazoness Quartet, to aid her in her goals. Nehellenia claimed to be the true Queen of the Moon, that she was the rightful owner and heir of the Silver Crystal and that the planet Earth belonged to the Moon Kingdom, of which she would be the ruler. She set out to get what she wanted, first by taking over the planet from within, by way of capturing the High Priest of Elysion, Helios, and corrupting the land of Elysion with her darkness, and then by sending the Amazoness Quartet to defeat the Sailor Senshi and retrieve the Silver Crystal from Sailor Moon. As a result of Nehellenia's invasion of Elysion, the captive Helios's spirit fled his body to seek out the maiden he had seen in a vision, whom he knew could help him. Helios told Sailor Moon and Sailor Chibi Moon that the only way they could save both Elysion and Earth was to find an item called the Golden Crystal, and awaken its power.
During battles with the Amazoness Quartet and their minions, the four Inner Senshi obtained their Sailor Crystals with the help of Artemis, Phobos and Deimos, and the Sailor Power Guardians. At the same time, Hotaru Tomoe grew in age, reawakening as Sailor Saturn and giving the other Outer Senshi their respective Sailor Crystals. Once the Sailor Senshi had managed to defeat all of the Dead Moon's minions, they finally discovered that the Golden Crystal was hidden inside Mamoru's body, much as the Silver Crystal was within Usagi's.
The Senshi then went to confront Nehellenia in Elysion, where she attempted to take the Silver Crystal one last time. The power of the Silver Crystal, combined with that of the newly awakened Golden Crystal, bound together by the love of Sailor Moon and Tuxedo Mask for one another, prevented this yet again. When defeated by Eternal Sailor Moon using the joined powers of all the Sailor Team, Nehellenia shriveled into a wizened old woman, and, after being sealed inside her mirror once more, vanished completely.
Queen Nehellenia is a a powerful user of black magic. She casts multiple curses which inflicted harm on Elysion, Helios, Mamoru, the Earth and eventually Usagi. She also cursed the infant Princess Serenity, declaring she would not live to ascend the throne and the curse appeared to be enforced by the Dark Kingdom, which destroyed the Silver Millennium. Like other incarnations of Chaos, she is able to corrupt other beings, such as the Asteroid Senshi whom she turned into her Amazoness Quartet.
- She is the only incarnation of Chaos that had a humanoid form. While Wiseman looked human, his true appearance was the planet Nemesis.
- It has been theorized that Queen Nehellenia is named after the Frisian-Roman goddess Nehalennia, who is affiliated with ships and seafaring and has been likened to "Nether Moon".
- She used "wareware" (我々; we) as a personal pronoun which is the japanese quivalent of the royal plural.
- She shares similarities with Queen Grimhilde from Snow White and the Seven Dwarfs. Both are obsessed with mirrors, and pride themselves on how fair they are.
- She also shares similarities with Maleficent from Sleeping Beauty as she cursed a newborn child out of spite with the king and queen.