In Assassin's Creed: Brotherhood, Ezio leads the Roman Assassins as their Mentor.
The title reflects Ezio's senior role in rebuilding the Brotherhood after the Borgia occupation of Rome. He recruits and trains new Assassins, assigns them missions, and coordinates efforts against Cesare Borgia and his allies. Brotherhood makes this leadership central to its gameplay, allowing players to expand the organization and call recruits into combat.
Ezio had already become a highly experienced Assassin by the time the game begins. His work in Rome is not simply another personal revenge campaign; it is an attempt to restore an organization whose network and influence have been badly damaged. The title therefore describes both his rank and his responsibility.
“Grand Master” is chiefly associated with Templar leadership in the series, while “Mentor” is the conventional senior title used for an Assassin Brotherhood leader. The game also depicts the Brotherhood as a wider organization rather than only a small group based in one city.