Caliborn

undyingUmbrage is likely a troll, unknown 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"

Speculation
uu is the antithesis of UU, who, in turn, is an antithesis of Karkat. This implies further association with Karkat. uu could very well be the pre-scratch Sufferer, given his ornery nature and the frequent capitalization in his typing quirk. The dark grey and bright red colors of his symbol may also relate to the Sufferer.

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.

The black and red coloration of his symbol, black text, and his strait and to the point nature may also point to some relation to His Honorable Tyranny.

Trivia

 * uu's hex code is 32, the atomic number of element Germanium, a brittle grey crystalline element used as a semiconductor. 3 is the difference of 9 and 6 (Karkat's hex code is 62; UU's is 92).
 * 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 the caduceus, also a symbol 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.
 * Like UU, uu also seems to be a representation of the fan community, particularly his impatience with the speed at which things unfold.
 * However, he's also a representation of people who don't like Homestuck. His complaints about the pesterlogs mirror common criticisms about the comic. 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 color scheme (black and red) as well as his plans to "puzzle murder" everyone are reminicent 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.