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Sinjin (or Saint John) is the son of Craig Kirk, the youngest son of George Samuel Kirk, Jr., and has one sister. The family grew up in rural Iowa where Craig worked as an agricultural engineer. For a time Sinjin followed his father in studying to becoming an agricultural engineer but eventually developed an interest in joining Starfleet and despite his father's objections applied to the Academy.
In his third year at the Academy Sinjin became Squad leader, he studied military history and excelled in battle tactics classes. In that year he scored the highest scores on the Kobayashi Maru scenario in the Academy's history.