Also known by his Trollian handle carcinoGeneticist, is one of the Trolls and is one of the main characters in Homestuck. His associated zodiac sign is Cancer (♋) and he has horns that are rounded instead of pointed (constantly referred to as "nubby horns" or "Candy Corn.") He types in gray with all caps and near-perfect syntax.
Karkat is a crab in more ways than one. He constantly insults others in the style of a stereotypical internet troll, supporting his arguments with what seems to be empty logic. In keeping with the crab theme of his personality, he rarely smiles. He is also often seen screaming, pounding his fists against his keyboard, or generally raging when talking to anyone. He is not particularly patient when explaining himself, making even his most sound advice appear to be random insults.
Despite his flippant demeanor, he also seems to care quite deeply for his friends' well being, judging from his tearful reaction to Sollux's apparent death, his seemingly genuine desire to help Eridan and Tavros with their relationship issues with Feferi and Vriska, respectively, and his willingness to revive Kanaya.