Caliborn

undyingUmbrage is likely a troll 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.

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.

Role in Troll Society
uu is the antithesis of UU, who, in turn, is an antithesis of Karkat. This implies further association with Karkat. His ornery nature, tendency to ramble, his typing quirk, and the bright red in his chat symbol all support a possible connection to Karkat. He may be the pre-Scratch Sufferer. This could be the case due to the pattern of connections between the universes (the post-scratch trolls' universe corresponding with the pre-scratch kids' as well as the pre-scratch trolls' corresponding with the post-scratch kids'). Unlike Karkat, uu wears his blood color on his sleeve by including it in his symbol. This could further imply uu has mutant red blood.

Another possibility, however unlikely, is that UU and uu are two halves of one troll, a pre-scratch Sufferer. 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).

The black and red coloration of his symbol may also point to some relation to His Honorable Tyranny.

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. Doc Scratch 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. However, if uu really is a Hero of Time, then that would mean he is also the Knight of Time, since each session is implied to require a Knight to assist the hero of space with the frog-breeding duties.

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 japanese symbol for Ophiuchus, whereas UU's is a hybrid of the caduceus and rod of Asclepius, the traditional Greek symbols for Ophiuchus
 * The red in said symbol is the same hex code as Karkat's blood color (#FF0000). What this says about his blood color is unclear still. As stated by UU, however, it is at least a different color from her.
 * 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.
 * 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".
 * According to Dirk, uu has requested Dirk to draw porn for him before. uu has not denied this and even seems to have HS 6547 (confirmed it).
 * 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.