Calliope

uranianUmbra is an alien player from an unknown session of the game. Clues in the story currently suggest she is female, and a thirteenth troll player.

She might be considered Jane's or Jake's patron troll, 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").

At the end of the HS 6013 ([S] ACT 6) flash, a terminal with a caduceus symbol can be seen, from which it is speculated that this is 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. However, it is not certain whether these hands belong to UU. 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.

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 off of.

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.

The most popular supposition is that uranianUmbra is a 13th troll player, associated with the zodiac sign Ophiucus, as she types in a gray tone, similarly to Karkat Vantas it was speculated that, like him, she's got a mutant blood color.

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, and if there is still a relation to Karkat Vantas and her blood is somehow also mutant, it could be bright blue, in-between two blue bloods, and corresponding to 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 spelling (probably so that she can use the letter U more), and a degoratory term for a British person is 'limey', it does seem quite possible that she is indeed a limeblooded troll. 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)).

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.

Given 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. In addition, 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.

If UU is a pre-Scratch troll, she is likely The Dolorosa, Redglare, or Mindfang, since these were the female ancestors of Prospit dreamers. While the post-Scratch Handmaid or Condesce could be pretending to be UU at the behest of, this is very unlikely as she would have to be lying to the point of character fabrication.

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 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 malliable, or a matter of choice.

In addition to this, Pre-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. Thon convince 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.

It is also possible that the exile-style terminal is being used by the Batterwitch to manipulate Jane and Jake. Her friendly and flirty nature has some things in common with Feferi Peixes's writing style; it is known that, when meeting new races for the first time, The Condesce acts is a friendly manner, which only softens them up for the coming armies. This would reflect a third "prong" in her attacks against the heroes, in addition to BettyCrockerCorp and her takeover of Derse and exploitation of its Agents..

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).