Karkat Vantas is one of the main characters of Andrew Hussie's webcomic/multimedia work Homestuck. With the completion of Homestuck, I can now write about the characters, and Karkat is perhaps the best place to start, since he is possibly the most important character in Homestuck. There will be major spoilers for the plot and character arcs of Homestuck below.
Karkat was first introduced to readers as an anonymous voice speaking to the main, human protagonists of the story over an instant messaging program, and is first a very abrasive, antagonistic character who "trolls" the heroes of the story. Over the course of thousands of pages of the story, or over a year of real time, Karkat is revealed as a kind and helpful character who hides his concern for others behind a foulmouthed persona. Karkat and his eleven friends are revealed to be a group of aliens who found themselves in the same deadly predicament as the heroes, and who finally decide to work together with them, an alliance that quickly turns into a confusing series of relationships. Karkat's journey from one note joke based around his CONSTANT SHOUTED INSULTS TO EVERYONE to a warm, multidimensional character parallels the development of Homestuck and its audience from a piece of random, "outrageous" internet humor to a nuanced character study.
Within the dense, and sometimes ironic mythological and symbolic system of Homestuck, Karkat has several roles. Him and his eleven friends represent the twelve signs of the Zodiac, with Karkat, as his name suggests, being the sign of cancer. His species has a strict biological caste system, but Karkat himself is (as his name suggests) a mutant, standing outside of the caste system. Many jokes and puns about cancer are connected to him, from his constant attacking of himself, to his "crabby" disposition. Furthermore, Karkat is revealed to be The Second Coming of Jesus in his culture, and much of his later character development focuses on the mild, pastoral type of leadership he shows beneath his angry exterior. It is a credit to Homestuck that despite the seemingly "wacky" nature of this paragraph, a character who is both the embodiment of cancer and the second coming of Jesus is not played off as something outrageous or weird, but is a natural part of his character development.
It is also interesting that despite the seeming blasphemy of having a foul mouthed Jesus figure, Karkat is closer to the biblical Jesus than most allegorical portrayals of Jesus. Instead of having a fantasy portrayal of either a noble warrior king or a somber wiseman, Karkat seems to capture the Jesus who sarcastically outwitted his enemies, cursed a fig tree, and (as Homestuck points out) died cursing. Although jarring at first, it is actually, as far as we know, historically accurate, and literally fitting.