Desmond Miles is the Assassin’s Creed character who appears as the central modern-day protagonist across the most early mainline games.
Desmond is introduced in the first Assassin’s Creed as a bartender descended from important Assassin ancestors. Abstergo Industries abducts him and uses the Animus to access his genetic memories. Through those memories, players experience the lives of Altaïr Ibn-La'Ahad, Ezio Auditore and other historical figures.
His modern-day storyline continues through Assassin’s Creed II, Brotherhood, Revelations and Assassin’s Creed III. Desmond’s role is unusual because he is not the historical character controlling most gameplay, yet his choices connect the separate time periods and drive the overarching early saga.
Later mainline games change their modern-day format and introduce different framing characters. That is why counting only historical playable heroes can produce a different answer. Desmond is specifically the recurring modern-day protagonist of the franchise’s first major story arc.