Which Assassin's Creed protagonist is known as the Mentor of the Assassins in the 12th century?

Altaïr Ibn-La'Ahad is known as the Mentor of the Assassins in the 12th century.

After the events of the first Assassin's Creed, Altaïr rises to become Mentor of the Levantine Assassin Brotherhood. In that role, he reforms the order, studies the Apple of Eden, and helps spread Assassin influence beyond the original fortress at Masyaf. His leadership is explored most fully in Assassin's Creed Revelations and related material.

Altaïr's career begins with a serious failure during a mission in 1191. Al Mualim demotes him, and Altaïr must complete a sequence of assignments before recovering his rank. This early plot is distinct from his later achievements as Mentor, which is why the title does not describe his position at the beginning of the first game.

Ezio eventually encounters Altaïr's legacy in Constantinople, while Bayek is associated with the founding of the Hidden Ones and Edward with the Caribbean Brotherhood. Those characters are important leaders, but they are not the 12th-century Levantine Mentor identified here.