There are two mentors in Anno 1404: Lord Richard Northburgh for the occident and Grand Vizier Al Zahir for the orient. These non-playable characters always have a trade agreement with you and cannot be attacked or destroyed.
During the campaign, they not only serve as central story characters but serve to guide and teach you how to play Anno 1404.
Note that these mentors are distinct from computer players, which play as you do and try to win the game. The mentors are also distinct from neutral powers since the mentors are always allied with you and are on your side no matter what.
Also note the Corsairs faction, led by Hassan ben Sahid, is a hybrid group that acts partially like a computer player who attacks you and demands tribute, partially like a neutral power since he'll offer you benefits like ships for sale if you meet his requirements, but also like a mentor since he has his own small island and you can eventually sign a permanent "fraternization" agreement with him that makes him a perpetual ally and trading partner.
The Venice expansion adds a third mentor, Giacomo Garibaldi, who introduces you to the new espionage game elements and also acts as a pepetual trading partner.