Knave is a word often heard in stories that are based on medieval Europe. It is a way of calling someone dishonest that no one uses anymore because if they do they will end up sounding extremely posh. An interesting thing I noticed when googling this word to find a definition is that knave wasn’t always a negative word, originally a knave was a humble man or a male servant, then the meaning turned negative, then the word just stopped being used.
A knave character is a character who no one can trust. They will lie more than anything else. This is a type of character you can use to add interest to your story because you can never know if they are telling the truth until their lie is revealed. To write a simple knave character just make sure that they rarely tell the truth. That’s it, it’s that simple.
However if you want your character to even be remotely developed you need to think of a reason as to why they lie so much. Could it be that they are a pathological liar? Are they hiding something from the other characters and the reader that they must constantly lie to hide? Having a character lie all the time just for the sake of adding interest to the story is not a good idea. Making your character do anything just to add interest is also a bad idea. Your characters need a reason and a purpose for everything they do even if it is merely to relieve chronic boredom.