Daniel Cross is a Templar Agent sent into the Assassin Order as a Sleeper agent. Under Warren Vidic his ancestral memories were explored and Vidic used an Apple of Eden to give him a hidden purpose: to find and kill the Mentor of the Assassin Order.
Daniel could relive the memories of his Russian Assassin Ancestor without the use of an Animus. This means he was fairly unstable mentally because of the Bleeding Effect from his early childhood exposure to the Animus by Vidic. This means throughout his life he had been plagued by visions from those animus sessions. This hallucinations are the reason that Cross turned to drugs and alcohol to escape them. It was because of some late night drinking that he got, literally, pulled into the Assassin Order. This set him on his path to find and kill The Mentor.
He was eventually successful and then brought back into the Templar fold. He later became a master templar but showed some signs of addiction to the Animus. It would have a calming effect on him and cause the hallucinations to completely subside for a time.
Daniel Cross was the Templar who initially tracked and captured Desmond Miles for Vidic. The two of them met 3 more times: In New York, in Brazil and in Rome. During their Confrontation in Rome, when he had a chance to kill Desmond, Cross suffered an episode from the Bleeding Effect and started speaking in Russian. This gave Desmond the time he needed to overpower Cross and kill him.