Caliborn

undyingUmbrage is a troll with HS 6862 (cherry red blood) from an unknown session of the game.

uu's symbol is a variant of the astrological symbol for Ophiuchus, different from uranianUmbra's (albeit the more traditional one). Unlike the symbols of all trolls except UU, his symbol contains two colors, a dark grey "U" with a red tilde (~) across it. He seems to be the mentioned "brother" UU is working with, and is generally an antithesis of her. He appears to be ornery (which contrasts UU's politeness). uu's typing quirk is also the opposite of UU's, capitalizing every letter except u. Like Karkat and UU, he types in a shade of grey, having abolished "hemotyping" as part of a game he has been playing with UU for most of his life.

He is allegedly a Derse dreamer, due to UU mentioning that she is a Prospit dreamer and that her brother must enjoy his "Netherworldly affiliation."

According to uu, the trolls (or at least he and UU) have a sort of superstition involving Lil' Cal, which suggests that any person close to or even occupying the same plane of existence as Cal, or any "jujus" like it will have a nightmarish life. This superstition probably has to do with another character who has many of the same features as Lil' Cal. Apparently the fear associated with this superstition isn't debilitating however. The use of the term "juju" might also imply he recognizes Cal's origin as a Subjugglator-created Chucklevoodoo.

Personality
uu has a terrifying obsession with playing games, so much so that UU claims his love for them surpasses human comprehension of what love is. In addition to the larger game between him and UU, the two regularly play chess, with UU winning every game except one. He also played a game with Dirk Strider in which he threatened to kill UU's dream self unless Dirk drew him pictures of the B2 kids in various romantic pairings. When Dirk won by making him too uncomfortable to keep playing, uu revealed that he had HS 6874 (already killed) UU's dream self before he started the game with Dirk. Despite this obsession, he is apathetic towards Sburb, seeing it mostly as a means of escaping his dead home planet. He still intends to play it, but he plans to treat the game as a competition against UU instead of the cooperative game it is meant to be, which could make winning the session impossible.

When playing his games, uu enjoys deploying "shitty" twists designed to nullify any lead his opponent might have or any reward they might get from playing. His game with Dirk used two such twists: killing UU's dream self before the game and not telling Dirk about it for an entire year. During a chess game against UU, he asked if he could swap his king and queen's starting positions, then left them in their normal positions but disguised as each other to trick UU into checkmating the wrong piece.

Relationships
He is the "brother" of uranianUmbra and has a conflicted, aggressive relationship with her and often threatened to kill her dreamself, an act he HS 6875 (eventually performs) by hiring his Session's Jack Noir, who then stabbed her.

He seems to have a liking for Dirk, as far as he is capable of such emotions, calling him the most tolerable out of the B2 kids.

Role in Troll Society
uu is the antithesis of UU, who, in turn, is an antithesis of Karkat. This implies further association with Karkat (except while Karkat hates mostly himself, uu hates everything else). His ornery nature, tendency to ramble, his typing quirk, and their shared blood color all support a possible connection to Karkat.

A common theory is that UU and uu are two halves of one troll, in a relationship somewhat akin to Jekyll and Hyde, with one being the exact opposite or yin to the other's yang like in the cancer symbol (UU said she had to go to sleep and then uu became active). This is further supported by UU's conversation with Jane in LOCAH, when UU states that after a "dreadfUl nap" she "woke Up to find [her] coplayer has made a terrible mess here." This prompts a string of questions from Jane which UU is uncomfortable with, including whether or not she and uu have met. Also notable is the fact that UU and uu seem to be incapable of simultaneously cheering/jeering one another. They also appear to be playing chess together on one board, and yet are only able to infer, rather than observe, one another's actions (such as UU suspecting that uu is HS 6932 (possibly eating the chesspieces)).

Role in the game
If uu and UU really start and remain in a two player session, then it is likely that uu is a Hero of Time, because the time aspect seems to be one of two essential aspects to a session. said each session will have a Beat Mesa equivalent and that it will be in the Hero of Time's land. This would also suit the speculation about UU being a Hero of Space, the other seemingly essential aspect of a session. Furthermore all Heroes of Time we have seen so far have been Derse Dreamers, just like uu. If uu really is a Hero of Time, he may be another Knight of Time, since each session so far had a Knight assisting the Hero of Space with the frog-breeding duties. It's however unclear if this is a session constant, and where it would leave the combination of the "Knight of Space". In HS 6550 (this page), Dirk refers to uu as having "stolen enough of [his] time." This may foreshadow uu's status as a Thief or Rogue of Time.

Calmasis
Continued revelations concerning the Complacency of the Learned books suggest that UU and uu are two parts of the same being. In Dirk's conversation with Jake in 2008, the discussion of the books brings up an image of Calmasis bound by two shackles, one bearing the Caduceus symbol of uranianUmbra standing on red ground and the other bearing the lesser-known Ophiuchus symbol of uu on green ground. It should be noted that the more common Ophiuchus symbol is often confused with the Caduceus symbol. Andrew Hussie may have been using this to imply that they are represented by the same astrological sign, which happens to be the 13th sign of the Zodiac in some cultures, which adds to the theory that the two trolls are one. In this picture he/she appears to have a particular deudly firearm, leading to speculation that uu/UU may be Lord English's ultimate downfall.

Trivia

 * uu's hex code is #323232, 32 being the atomic number of element Germanium, a brittle grey crystalline element used as a semiconductor and anemia treatment. 3 is the difference of 9 and 6 (Karkat's hex code is #626262; UU's is #929292). In addition, 6 and 9 are both multiples of 3.
 * The word Umbrage can mean an offense or an annoyance. It can also refer to a shadow, likely a nod to the Umbra portion of uranianUmbra.
 * uu's symbol is the symbol for Ophiuchus proposed by British astrologer Walter Berg in 1995, which gained widespread popularity in Japan, whereas UU's symbol is a hybrid of Caduceus and the rod of Asclepius.
 * The red in said symbol matches his blood color which is the same as Karkat's blood color.
 * uu "jeers" instead of "cheers", opposite to UU.
 * uu seems to represent different aspects of the Homestuck consumer community from UU, particularly his impatience with the speed at which things unfold. As many less fanatic readers do, he complains about long pesterlogs formatted strangely in unusual colors. UU's relationship with him mirrors the fans who keep pestering their friends to read Homestuck, even though they're not interested or don't "get" the story.
 * His denial of being related to UU by the human construct of siblinghood seems similar to people who criticize Homestuck fans describing their relationships with quadrants.
 * uu's "tumut" emote is one letter from "tumult", which means chaos or disorder, possibly a reference to his rather aggressive personality as well as his handle.
 * His color scheme (black and red) as well as his plans to "puzzle murder" everyone are reminiscent of those of Jigsaw from the movie "Saw".
 * uu is the only character in Homestuck to have lowercase for both letters of his screen name.
 * uu could be a reference to a very old forum post, in which someone bothers Andrew to draw him muppet pornography while in the guise of Jigsaw. It's also interesting to note that the two proceed to draw comics back and forth, and one of the comics that Andrew draws has actually been seen HS 2465 (in-comic) as something that Bro drew. The thread can be found Here.
 * Karkat Vantas' typing color is directly in between that of UU and uu's.
 * His attitude and UU's could be a reference to the Sufferer. It is stated that the Sufferer was first kind and gentle, much like UU, but HS 5955 (during his penance) all the kindness he had turned to anger and hatred, much like uu. He also shares his blood color with The Sufferer and Karkat.
 * He has also been shown to have HS 6880 (slips of tongue) much like Karkat when using HS 5771 (figures of speech).