At first Saduj didn't say much. However, after the Avatar revealed to be a member of the Oppression by saying the password "IMPERA", Saduj became much more talkative. Saduj was a spy sent by Lord Blackthorn to get possession of the Sandalwood Box, to ensure that Lord British would be sealed in the dungeon Doom permanently. He then offered his services to the Avatar's party.
Should the player let Saduj join, he will betray the party in combat as soon as possible.
Amusingly, one can let Saduj join the second character space; and then make one's way to Blackthorn's castle without entering into a battle. Getting captured then means that Blackthorn will execute him, and Saduj's ashes will appear in the Codex.
Appropriately, "Saduj" is Judas spelled backwards.
Even worse, in Lazarus, Saduj steals the Sandalwood Box and flees, meaning an instant game over.