Correct me if I'm wrong, but, the reason Karkat has a sign is because he's the Signless's descendant. If we look back, the symbol Karkat has is the Symbol of the Signless. The Lusus karkat also had was pre-chosen for the descendant, as well as what his symbol would be. He has this sign because people believed he'd be the pone to carry on the Signless's message.