In Assassin's Creed Revelations, Ezio Auditore holds the title of Mentor.
By the time of Revelations, Ezio is an older and highly experienced Assassin. The title Mentor identifies him as the senior leader of the worldwide Assassin Order, rather than simply a field operative. His journey to Constantinople begins as an investigation into Altaïr Ibn-La'Ahad's legacy and the secrets held by Masyaf.
Ezio's role reflects the progression of his character across the Renaissance trilogy. In Assassin's Creed II, he is newly drawn into the conflict; in Brotherhood, he becomes a leader who organizes Assassins in Rome; in Revelations, he acts as a teacher and custodian of knowledge. The game also brings his story into contact with Altaïr's life.
Other titles belong to different protagonists or contexts. Bayek is a Medjay in Origins, while Kassandra is known as the Eagle Bearer in Odyssey. Grand Master is generally a Templar leadership title, so it does not describe Ezio's position in the Assassin Order.