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 seems to 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 he/she/it would be a blue blood, and probably pretty temperamental, her text color is gray. The panel after the start of Act 6 animation shows a troll, presumedly her, to be reading Alternian text, shown HS 6014 (here). It says "Click ==> to continue."

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.
 * 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.
 * Judging by their demeanor and the pattern of nicknames based on double letters (such as GG and TT) all being used by females, 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, like Jade and Kanaya, is a young Prospit Dreamer, having dreamt of Prospit for most of her life. Thus, she is likely a hero of Space.
 * Ironically given her chumhandle's ties to Uranus, the Caduceus symbol tied to Ophiuchus is also associated with the god Mercury.