Standard is a clone of Mitsurugi that is found in the source code of Soul Blade and can thus be played via cheat device. His movements are much slower than Mitsurugi's and he lacks his ponytail. He was most likely a test character rather than a proposed secret character.
However, it is known that Arthur was once planned as a guest fighter and did not get fully implemented. It is possible that 'Standard' was cloned so he could be edited to create Arthur but it never happened. There is also some fans that based on a concept art for the game of a samurai-like character with skull face believe that this was in fact Yoshimitsu initially planned to be on the game but later was dropped.