Clay Kaczmarek
Subject 16 or Subject Sixteen was a subject used by Abstergo Industries for the Animus project. He was the predecessor to Subject Seventeen, Desmond Miles. There is a possibility he may be related to Desmond, as their DNA match in AC2. It is also revealed in AC2 that he had an ancestor at the Battle of Gettysburg and in the Roman forum.
Assassin's Creed
Lucy reveals in one of her e-mails to Vidic that Subject Sixteen has Far Eastern origins, as one of his ancestors was "involved in an important event" there. A different e-mail hints that his name is "Neumann".
At the end of Assassin's Creed, due to the bleeding effect of the Animus, Desmond is able to use Eagle Vision to view many cryptic messages drawn in blood. Subject 16 states in the triangle shaped message on the floor that he drained his own body to paint the figures on the floor and walls of the main room where the Animus is located. But had he also used his own blood to paint the wall mural in the sleeping room, he would have bled out - literally - before he finished his work. It is possible that it is Leila's blood used in the wall mural in the sleeping room, done before he painted the messages with his own blood in the Animus room - a murder/suicide. Subject 16 probably attempted to kill himself but judging by the ruthlessness of those two men in suits (who are probably templars) they might have tortured him.
Assassin's Creed II
Subject Sixteen makes his first vocal appearance in Assassin's Creed II, making commentary about a set a 20 encrypted files that he hacked from the extensive Abstergo data banks. The said files contain what he claims to be "The Truth" about the origins of humanity. The man and woman in "The Truth" video, may in fact be Adam and Eve, activating, or "eating", "the apple", which is in fact, a Piece of Eden.
The files, as well as commentary from Lucy, reveal a bit more about the extent of the madness which lead to Subject 16's death: the Bleeding Effect from overusing the Animus. More specifically, the sheer amount of time he was forced to spend in the Animus. Lucy even saying that they forced him to stay in it for days at a time. As such, the number of genetic memories he was forced to unlock overwhelmed him and finally killing himself implied from the 20th encrypted file saying " Lucy, I can't wait any longer, I am ready to go. I hold the knife over myself, Lucy see's me."
Theories
It is possible that Subject 16 is also a descendant of Altair, but from a differet family than Desmond since Altair had at least two sons, as seen in one of the codex pages. Also, when Lucy and Desmond escape Abstergo, Lucy uses the Animus on Desmond one last time to search for any genetic memory matches between Desmond and Subject Sixteen, and finds that the memory of Ezio's birth is one they have in common.So, it is believed that Ezio is the link between Desmond and Subject Sixteen.