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.

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 aphroisms 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." 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.

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.

If uranianUmbra is a 13th troll player of Sgrub, and if her associated Zodiac sign is Ophiuchus, she might be located in the blue section of the hemospectrum between Vriska Serket and Equius Zahhak, or she may instead have a mutant blood color like Karkat Vantas (whose place in the hemospectrum is not denoted by the location of his constellation). 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)). Another possible color is the pure green blood which Hussie refers to as Lime Bloods. As she uses British spelling, and a degoratory term for a British person is 'limey', it does seem quite possible that she is indeed a limeblooded troll. It should also be seen that the snakes on the staff of her sign are in fact, lime green. It would also match her handle as Uranium usually appears silvery grey, but in some forms appears to be bright green.

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 Lord English, 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 is possible that gender is either malliable, or a matter of choice.

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. 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.

UU may be the Condesce or Betty Crocker, masterfully manipulating the kids of the Alpha timeline.

Trivia

 * UU has yet to reveal to Jane and Jake UU's real name, suggesting to Jane that it "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, 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.
 * UU's text color is defined by the hex code #929292; 92 is the atomic number of uranium. Uranium is naturally silvery grey, but in some forms appears bright green, similar to the two colors of UUs chat symbol.
 * An umbra is the darkest part of a shadow, it is also latin for shadow.
 * The beginning of UU's HS 6044 (conversation with Jane Crocker) is a direct parallel to Karkat's HS 2759 (CONVERSATION WITH JADE) which was revealed to be about her prototyping of Bec. HS 4039 (This Aradia-Kanaya conversation) also parallels that conversation.
 * 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.
 * UU is the only character that uses British spelling, such as the characteristic U in certain words (favour, honour, colour, etc.). This is obviously done in order to give UU more opportunities to use the letter U.
 * 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 are a unit of radiation).