This ghost blends in with the live handymen seen repairing the house, but it's revealed that he is a ghost, too. Steven Crain wrote about this clock repairman in his novel, but always believed he was a real man and claimed to never see any ghosts when he lived in Hill House.
His father later explained that the special antique clock couldn't be repaired by an average handyman, and he never hired anyone to work on it. This proves that Steven did, in fact, see ghosts in his youth.