Maybe the voice in the trailer was the Shepard VI? 
Anyway, depending on when the game takes place, I think Shep is best handled in a codex entry:
“The Shepard” is a socio-religious entity, worshiped in several belief systems with various degrees of popularity throughout the galaxy. While the fact that “The Shepard” represents an actual historical figure is widely accepted among the archaeological and historian community, some of the more miraculous deeds, ascribed to the figure (such as resurrection, the curing of entire races from sickness or “The Shepard’s” alleged death to atone for the galaxy’s sin of creating AIs) are disputed by scientists. A number of more radical critics have dismissed claims that “The Shepard” existed entirely, based on the fact that it cannot even be established if it was a man or a woman.
Interestingly, descriptions of “The Shepard’s” achievements are mostly consistent throughout the different religions that focus on this persona. However, the various interpretations differ when it comes to specifics, such as details in certain actions, exact words spoken and most importantly the general demeanor of “The Shepard”. While some ascribe a benevolent and compassionate stance to the figure, others uphold the view of an uncompromising and unscrupulous pragmatist. Frequently, these disputes escalate into violence between the different sects, the most notable recent occurrences being the Paragonian Crusades in the 31st century (Shepardian calendar) and the Renegade terror campaigns in the 3350s.
However, despite these schisms, most of the various clergies view “The Shepard” and the 19 apostles as messianic saviors and many await the return of a new incarnation of this mysterious entity.
