Earthborn - Raised on the streets, by the streets, Shepard joined up with multiple gangs as he grew to adulthood. Finding a home amongst the racist Tenth Street Reds. Joining the Terra Firma paryty, he imagined being a soldier as a way to fight for humanity (and show aliens who was boss), rather than himself. This was a fundamental change to his worldview at 18, and resulted in him becoming less heavy-handed towards other( human)s and more apt to act on faith.
Ruthless - His reputation for ruthless efficiency makes your fellow soldiers wary of him. But when failure is not an option, the Alliance can depend on him. Shepard sent 3/4ths of his unit to die and murdered surrendering Batarians on Torfan, judging that soldiers give their lives to the military and no soldier has the authority to surrender.