The Writer's Block is a personification of a lack of ideas. Its appearance is that of a block with cat ears, paws, and tail with its name on the side. It appears in filler comics to torment Dan (the character) when Dan (the author) is suffering from writer's block, though it has also appeared once in the main story, if only in Susan's mind.
For the most part, Writer's Block acts like a cat, though it moos as well as mews, and only eats plain veggi burgers. It has made 14 filler appearances and one story appearance. Besides causing problems for Dan, it also has a slight antagonism with the Demonic Duck because the Writer's Block considers himself superior to the Demonic Duck since he actually causes fillers whereas the Duck only distracts people in them. But, as the Duck rightfully points out, people actually like the Demonic Duck.
It is implied that in Dan's personal dimension the Writer's Block is a member of its own species, one of which Dan keeps as a pet. The Writer's Block has been seen to be able to fly. It has been observed to speak its own language, dubbed "Block-speak", but its thoughts are observable by other characters in Dan's dimension, including the Demonic Duck.