Which Assassin's Creed protagonist is known as the first Master Assassin of the series?

Altaïr Ibn-La'Ahad is known as the first Master Assassin of the series.

Altaïr is the central historical protagonist of the original Assassin's Creed, set during the Third Crusade in 1191. At the beginning of the game he is already a senior Assassin, but a failed mission causes him to lose his rank and weapons. His mentor, Al Mualim, orders him to complete a series of investigations before restoring his status.

The title “Master Assassin” describes Altaïr's position and later achievements within the Brotherhood. It should not be confused with the series' first playable protagonist in release order as a modern character: Desmond Miles frames the historical story but does not live Altaïr's medieval life.

Altaïr's codex and reforms also influence later Assassin generations, especially through the storyline of Ezio Auditore in Assassin's Creed II.