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 there 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
He is responsible for writing, with his own blood, the cryptic messages on the walls and floors of the laboratory which can be seen by Desmond at the end of the first game once he gains the ability to use Eagle Vision outside of the Animus. 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.
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".
Assassin's Creed II
It has been revealed that Ezio was the link between Desmond and Subject 16. The reason: Ezio sires a child or children that are in both Desmond's and 16's ancestral lineage (Since Ezio is a descendant of Altaïr this means that Desmond and 16 also share Altaïr as an ancestor). Subject Sixteen also 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.