Usually a young male, although shows acts of chivilary and kindness, usually has little regard for rules and regulation, and adopts a roguish attitude to his ventures. Often the antagonist of a White Knight.
"That guy just rescued 30 people from a bank siege! After he beat up the criminals, he found out the bank manager was the one who organised the holdup, so he kicked his ass and stole his wallet and car! That guy was such a Black Knight"