Calliope

uranianUmbra is an alien player from an unknown session of the game. Clues in the story currently strongly suggest she is female, and a thirteenth troll player. She appears to have grey skin and the typical orange horns, in a distinct wavy shape resembling the two snakes of her chat symbol. At least one of her sleeves is dark green and features a bright red 9, 6, or half of the cancer sign.

She might be considered the patron troll of all the post-scratch kids, based on parallels between her initial conversation with Jane and Jake and other initial conversations of kids with their patron trolls. In particular, the HS 006044 (initial exchange) between UU and Jane is a near-perfect antithesis to the HS 002759 (initial exchange) between Karkat and Jade (and, for that matter, the HS 004039 (initial exchange) between Kanaya and Aradia).

Her HS 006081 (conversation with Jake) includes details of her alien family structure that are consistent with what is known of troll development. As with her previously revealed HS 006044 (conversation with Jane), she is referred to as a female alien, and uses aphorisms characteristic of female gender in Western culture (e.g. calling the kids "darling" and "lovely").

She later admits to Dirk that she is fan of their story and near obsessed with them and also admits to having written fan fiction, including that of the romantic kind, about them.

At the end of the HS 6013 ([S] ACT 6) flash, a terminal with a caduceus symbol can be seen, which also serves as her chat symbol and may be her personal symbol. HS 6014 (The subsequent panel) shows a pair of troll-like hands, and the screen displays Alternian text that reads "Click ==> to continue" backwards. The terminal appears similar to the Homestuck Interface Devices used by Exiles, except with only one screen.

Her identity and whereabouts are currently unknown. Her session of the game may be separate from all four known sessions (the kids and trolls' pre-Scratch and post-Scratch sessions), but it is also possible she is one of the as yet unseen pre-Scratch troll players: the alternate-universe versions of the trolls' Ancestors, who played the initial failed session. This is unlikely however, as the shape of her horns do not match those of any known Troll.

In her conversation with Dirk, she talks about her "brother", who is supposed to be her client player, although she has offered to be his client instead. She stresses that he is not her brother in the sense that humans would understand it, and they do not even have the same blood colour, but that they have genetic similarity. This is consistent with much of what is known about troll biology.

She reveals that her mythological role in her iteration of the game was one of the HS 6414 (various classes always assigned to female players).

Speculation
The symbol associated with UU resembles a caduceus (☤): a staff entwined by two serpents, which is sometimes crowned with wings. The caduceus is one of the symbols of the Greek god Hermes, the messenger of the gods as well as the guardian of boundaries and the travelers who cross them.

By association with Hermes, the caduceus represents commerce and negotiation, two domains shared by Hermes and his interpretatio romana Mercury. In North America, the caduceus is commonly and mistakenly used as a symbol of the medical trade due to its similarity to the staff of the Greek god of healing and medicine Asclepius (⚕): a staff wrapped by a single serpent. In Greek myth, when Asclepius died, his body became the constellation Ophiuchus, the Serpent Bearer.

When the constellations were redefined in 1930 as consisting of certain areas in the sky, Ophiuchus along with Cetus the Whale now both crossed the Earth's ecliptic, but only to a small degree while the 12 constellations associated with the zodiac all do. Based on this constellation drift, a lone astrologer in 1970 suggested that the "true" zodiac should consist of 14 signs, one for each constellation that currently crosses the ecliptic, rather than evenly dividing the sky into 12 portions for the traditional zodiac. While this concept never caught on, Ophiuchus became popular as a thirteenth member of the zodiac in Japan after a famous Japanese astrologer proposed its addition to the zodiac in 1995. The suggested symbol for Ophiuchus looks like the letter U with a tilde in between (U̴), which UU seems to play with, particularly in emoticons.

It's possible that in UU's own session, she is a Hero of Space. She has mentioned to Jane that she has dreamt on Prospit all her life, an ability that thus far has only been possessed by Two other players, both Heroes of Space themselves. It may be that one of these players will in fact turn out to be UU later on, but this seems unlikely, since the first likely cannot read Alternian Text and the second currently has a skin tone that would appear to be a little too bright. It is arguable that her class will be different to any of the 12 seen so far, but if she does share the same class as another character, then the only options available would be Seer, Witch, Maid or Sylph. This is because these are the only classes that could so far be called exclusively female; Rogues and Thieves have been said to lean toward females, but not exclusively. Given her knowledge of Sburb, a Seer would seem a likely candidate, although if she were a Sylph, the possibility of another Sylph of Space may hint again toward someone relating to Kanaya

The most popular supposition is that uranianUmbra is a 13th troll player, associated with the zodiac sign Ophiucus. The additional constellation would also suggest some responsibility for bringing the kids' original session into existence, as her intercession also left a mark on the kids' universe (though it was HS 004524 (not shown with the other constellations)). Her horns support this theory, and she also states that she will be working with HS 006464 (someone she refers to as her brother), suggesting another troll made it through the reckoning, as a 14-sign constellation has also been proposed with Cetus as a sign.

Another possibility states that she's located between Vriska Serket and Equius Zahhak on the Hemospectrum, since Ophiucus is located between Scorpio and Sagitarius on the zodiac. On the other hand, as she types in a gray tone, similar to that of Karkat Vantas, it could be speculated that, like him, she has a mutant blood color. If so, one theory is that it could be bright blue, contrasting with Karkat's bright red blood.

A third possibility based on a answer Andrew gave to a question on Tumblr is having a lime blood, a position in the hemospectrum between Sollux Captor and Nepeta Leijon (where Karkat would be, if his blood color was defined by the location of his constellation). It should be noted that the snakes on her sign are lime green.

UU uses British/Commonwealth spellings and phrases, probably so that she can use the letter U more. There is an exception, "favor", in HS 6414 (this pesterlog), which is presumably Hussie's error (but you never know). One slang term referring to a British person is 'limey', which potentially reinforces the possibility of UU being a limeblooded troll.

UU's text color is also very similar to Karkat's: both are unusual choices of custom grays which differ slightly from the more common RGB-defined grays. In UU's color, the 6's from Karkat's color are replaced by 9's (#626262 becomes #929292), and Karkat's symbol is a sideways 69. 92 happens to be the atomic number of uranium. Uranium is naturally silvery grey, but in some forms appears bright green, similar to the two colors of UU's chat symbol, also tying with the possibility of UU being a lime-blooded troll.

That the initial conversation between Karkat and Jade antithetically mirrors Jane's initial conversation with UU further suggests a potential relationship between UU and Karkat. If Karkat HS 5917 (gave the kids' universe cancer), UU, as a symbolic healer if associated with Ophiuchus and as the antitheses to Karkat's consequences, could possibly provide the cure.

If UU is a pre-Scratch troll, she is likely The Dolorosa, Redglare, or Mindfang, since these were the female ancestors of Prospit dreamers. The theory that the post-Scratch Handmaid or Condesce could be pretending to be UU at the behest of, already very unlikely as she would have to be lying to the point of character fabrication, has now been all but ruled out since the shape of her horns has been revealed.

However, she could also be the pre-Scratch version of the Sufferer if his gender were changed, given that the caduceus is also associated with various origin myths of the prophet Tiresias in which Tiresias changes sex from male to female and back again. The 36th Sweet Bro and Hella Jeff mentions a "gray mutant penis," which could indicate that UU's sex and blood color both underwent mutations into the post-Scratch Sufferer. Other speculations state that UU's relationship with Karkat implies that she is the leader of her session. One of her sleeves sports what could be half of the cancer sign in bright red, supporting a possible connection to Karkat.

The term "Uranian" is also an archaic term initially used as a label for gay and bisexual men, and later for individuals who are intersex or transgender. This term originated from a character in Plato's Symposium, who used the myth behind Aphrodite Urania to defend his mental and spiritual attraction to Socrates. This might tie in with the fact that there is no troll stigma against gay and lesbian romantic pairings and that while they clearly have gender roles, troll sexes have not been elaborated on. This makes it possible that gender is either malleable, or a matter of choice.

In addition to this, Post-scratch Rose wrote a book in which UU's associated symbol is shown on the cover. The book is said to be about "[an] androgynous young apprentice, Calmasis, who throughout the series plays the roles of antihero and chief antagonist. Calmasis convinces fellow disciples to rebel against Zazzerpan's vaunted Complacency, and one by one hunts down each wizard. All twelve are killed but the Predicant Scholar himself, forcing a showdown." This could possibly connect to UU being both genders. The little we've seen of UU's outfit sports the same color scheme as Calmasis' clothes, namely bright red accents on dark green.

Trivia

 * UU has yet to reveal to Jane and Jake her real name, suggesting to Jane that "it may be for the best that yoU never know it," because "it coUld stir Up some things best left in their present eqUilibriUm." This suggests her identity is of significance to Jane, and that she may be someone that a member of Jane's session has already met, or will meet in the future.
 * The 4 universes that UU refers to are: the instance of the trolls' universe in which the Ancestors played Sgrub (A1), the instance of the trolls' universe in which the current trolls played Sgrub (A2), the instance of our universe in which the pre-scratch kids played Sburb (B1) and the instance of our universe in which the post-scratch kids will play Sburb (B2).
 * Most likely, as the kids and trolls' pesterchum handles are combinations of the A, T, G and C bases of DNA, this troll's obsession with the letter U is a reference to uracil, a nucleotide base that RNA contains in place of thymine (T).
 * The astrological significance of the U may be a reference to the planet Uranus (debunked by UU herself when Jane assumed she was an alien from Uranus), or to one of the proposed symbols associated with treating Ophiuchus as a zodiac sign: a U with a tilde-like symbol superimposed across it as a stylized representation of a man carrying a snake. Since Ophiuchus is not a zodiac sign, there is no standard Zodiac symbol associated with it, but the U symbol has gained popularity in Japan.
 * Umbra is Latin for 'shadow'; in English, it can mean the same, but usually refers specifically to the darkest part of a shadow.
 * UU's client begins and ends with cheering, which is different from Trollian, Pesterchum and Bettybother which begin and cease with trolling, pestering and bothering for each of its respective clients.
 * There are two songs that could be clues to UU's identity: Unbreakable Union, and its remix: Umbral Ultimatum. Each song is played in a flash involving Becquerel, which could support UU's link to radioactivity (becquerels being a unit of radiation).


 * It is possible that UU could be a signless from the symbol on UU's sleve