Sir Hiss is a snake who serves as an advisor (and "man Friday") to Prince John.  While he is a master of the spoken word and of etiquette (befitting his role), he is also a very clever, conniving, scheming and sinister fellow, who is completely loyal to Prince John...almost fawning.  However, he is also very clearly the smarter and more observant of the two, even if Prince John constantly insults him and his intelligence.  If it were not for the prince's own ineptitude, it is quite possible that things could have been much tougher for Robin Hood.

Sir Hiss is also an accomplished hypnotist, using his eyes and his body as effectively as any snake would in order to achieve his nefarious ends.  It was he, at the urging of Prince John, who hypnotized King Richard and persuaded him to go off and participate in the Crusades, opening the door (temporarily at least) for Prince John to usurp the throne.

Origins of the character: Sir Hiss does not have any foundations in the Robin Hood legends.  While the character of Prince John would have had counselors and advisors (and supporters) in the many versions of the legends, Sir Hiss is pretty much an original creation.

It is possible, however, that Sir Hiss may be this story's interpretation of one of the bad guys in the Robin Hood legends, Sir Guy of Gisborne...though it is a tenuous connection.