John does not call Karkat 'Kar' Eridan does. Eridan Shortens everyones name to 3-4 letters in their name. Kar, Fef, Vris, Sol

Yes, this is true, but a friendly reminder that Eridan CAN also call people their full names too, and vice versa, with others calling them “Kar, Fef, or Sol.”

-Mod F