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.

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, however unlikely, is that UU and uu are two halves of one troll. 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.

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 deadly 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 reminicent of those of Jigsaw from the movie "Saw".
 * He has, in the past, asked Dirk Strider to draw smut for him. In exchange, he promised not to kill UU's dream self.
 * 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 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 when he was exiled all the kindness turned to anger and hatred, much like uu, as well as sharing blood color with The Sufferer and Karkat.