By day, mild-mannered Dexter is a blood-spatter analyst for the Miami police. But at night, he is a serial killer who only targets other murderers.
Sgt. Doakes makes comments to Dexter that after looking into his background, he found that Dexter studied Advanced Jujitsu in college (which would possibly equate to a black belt), and that Dexter finished at the top of his class in medical school, but chose blood spatter analysis as his career. This implies that Dexter is a non-practicing M.D., though it's never furthered explored or mentioned thereafter.
Dexter Morgan:
I've lived in darkness a long time. Over the years my eyes adjusted, until the dark became my world and I could see.
Visible throughout the first season, Dexter has a large scar on his left side. Later, in season two, the scar has moved to his right side, leaving his left side unmarked.
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