In Assassin's Creed Revelations, Desmond Miles is the playable character in the modern-day sections.
The main historical storyline follows Ezio Auditore in Constantinople, where Ezio searches for Altaïr's keys and explores the legacy of the Masyaf library. Desmond's modern-day storyline takes place largely inside the Animus, after his mind has become dangerously fragmented following the events of Assassin's Creed Brotherhood.
Clay Kaczmarek, also known as Subject 16, appears in Desmond's mental landscape and provides important context, but he is not the modern-day playable character. William Miles is Desmond's father and a senior Assassin, while Daniel Cross is a Templar operative. Revelations connects the three major protagonists of the original saga: Desmond, Ezio, and Altaïr.