Karkat Vantas

Introduced in Act 3, and currently known only by his Pesterchum chumhandle. An incredibly rude individual who trolls Jade, earning him a place in her Trollslum. He types in light gray. He is represented by the symbol ♋ in illustrations. He seems to be aware of Jade's activities and of the kids' plans to play a game. He also seems to want a different outcome, asking Jade "IS THERE NOTHING I CAN DO TO CHANGE YOUR MIND?" indicating he knows what Jade wants to do, and wants her to do something different. Thankfully, she outright denies.

He types in all caps and with less-than-perfect grammar, as is typical for an internet troll. He also constantly insults others, supporting his arguments with what seems to be empty logic. He is not particularly patient in explaining himself, making even his most sound advice appear to be random insults. CG has the ability to pester Jade even though he's been blocked and Jade is signed off. This suggests he is more intelligent than his diction supports.

His chumhandle is particularly nefarious sounding, implying genetic engineering and carcinogenic factors. This could suggest that one of his interests is intentionally causing cancer. Like others in Jade's Trollslum, his name has a zodiac symbol implied - in this case, Cancer the crab. Despite his rudeness, he seems to be broadly on the same "team" as the kids instead of acting as one of Homestuck's villains.

Alarmingly, his chumhandle name is quite similar to John's: EctoBiologist. CG may serve as a foil to John. This is supported by their mutual interest in movies. (even if they are staggeringly different in their own dimensions)

Real Life?
A person using the handle "CarcinoGeneticist" is also present on the MS Paint Adventures forum. He acts exactly the same and even types in the same color as the CarcinoGeneticist in Jade's Trollslum, and everyone is certain they are the same person. At the moment, very little has been revealed about him.

Since then, however, due to an alarming influx of such Troll accounts, pretending to be the trolls is no longer allowed. CarcinoGeneticist is still present on the forums, but is forbidden from using his trademark font. It would appear he is very pissed off about that part.

Midnight Crew Intermission
The kid Spades Slick messages is carcinoGeneticist, judging by his shirt, which has the cancer symbol on it, not to mention the sprite which appears to have crab-like features, or the fact that his passively-displayed "command" is prefixed by the initials "CG." He appears to be engaged in a game of Sburb, as he has a Kernelsprite, a Cruxite Dowel, and some sylladex cards in his room. His weapon of choice appears to be a sickle—presumably his Strife Specibus reflects this as either scytheKind or a previously-unseen sickleKind. This is most likely to contrast or even complement John's hammerKind, as the hammer and sickle are the symbols of communism, and thus linked. It should be noted that he strongly resembles a classical troll.

Act 4
CarcinoGeneticist later contacts John, and tells him that they will be friends in the future. He also implies that time seems to move in the other direction for him and his Trolls than for the Homestuck Kids, but in reality his trolling schedule is simply disorganized, unlike that of the linear GallowsCalibrator. His peculiar timeline may be a reference to his sign, the crab, which is associated with non-linear and sideways motion. This seems to explain a mysterious remark GrimAuxiliatrix made to Rose.Later on, he contacts John once again to ask him what he is doing and they have a discussion on movie title, and it is implied that CarcinoGeneticist's civilization is much older than John's since they ran out of good movie titles over thousands of years. John also reminds CarcinoGeneticist that he wants to talk to Jade and to tell him to calm down if he contacts him any earlier in the timeline so as to prevent any time paradoxes from occuring. He also finds out that John is in "cahoots" with GallowsCalibrator and gets pissed and asks John to send along a nasty message to her.