Calliope

Suspected to be the legendary 13th troll (although she may be a member of another unspecified species), uranianUmbra is the player in a session separate from those of the pre-Scratch and post-Scratch kids and trolls, and might be Jane Crocker's patron troll. This zodiac sign is only in the sky for a few hours between Scorpio and Sagittarius, which would mean she would be a blue blood. Her text color is a light gray; the reason for this is unknown, unless she is somehow another mutant-blooded troll like Karkat Vantas and his ancestor. Or, it might just be anonymous to keep us guessing.

At the end of the Act 6 flash animation, a terminal with her Caduceus symbol can be seen. The panel after the start of Act 6 animation shows a pair of troll-like hands to be reading Alternian text, shown HS 6014 (here). It reads "Click ==> to continue." It is unknown if these hands belong to UU, as the terminal appears to be similar to the Homestuck Interface Devices used by Exiles, except with only one screen.

Trivia

 * UU's status as a player in another session is possibly related to Ophiuchus's sign not being acknowledged as an official zodiac, like the other 12 trolls/signs.
 * The 4 universes that UU refers to are both the Kids universe and its alternate Scratch self, and the Trolls universe and alternate Scratch self
 * Most likely, as the kids and trolls' pesterchum handles are based off of the A, T, G, C of DNA, this troll's obsession with the letter U is based off of uracil, a nucleobase used in RNA.
 * The U may also be a reference to the zodiac symbol associated with Ophiuchus, which is a U with a tilde-like symbol superimposed across it. This symbol is a stylized representation of a man carrying a snake.
 * In Greek mythology, the God of Medicine and Healing Asclepius upon his death was turned into the constellation Ophiuchus. While his symbol was the rod of Asclepius (⚕), a single snake entwined around a staff, uranianUmbra is associated with the caduceus (☤; "the herald's staff"), which is a winged staff with two snakes entwined around it. The caduceus is primarily associated with Hermes, and by association Mercury, who was the messenger of the gods, as well as the patron god of boundaries and the travelers who cross them. The caduceus is generally related to commerce, but it has become synonymous with the rod of Asclepius in North America.
 * The "Uranian" part of her name may refer to the Greek deities Uranus, primordial god of the sky, or Urania, the muse of astronomy. "Uranian" is also a 19th century term for people of a third sex (homosexuals, transsexuals, and transgenders), which is derived from a part of Plato's Symposium. A character in the Symposium states that because there are two myths as to why the goddess of love Aphrodite was born, means that the one who was born of the severed genitals of Uranus cast into the sea by Cronus is the goddess of homosexual love, because she was not born of a woman.
 * An umbra is the darkest part of a shadow.
 * Judging by their demeanor, it could be assumed that UU is female, Jane also refers to her a girl, presumably based off previous conversations between further suggesting it.
 * The beginning of UU's 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. These parallels may hold meaning or simply be narrative callback.
 * 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.