How many/which characters in DEXTER found out that dexter is a serial killer?

dead or alive im on season 6 (i barley started watching it 1-2 months ago im on episode 6 so NO spoilers) please i just wanna know how many/which charatcers found out dexter is a killer?
Update: also does anyon know how much "Double Dexter" the book by Jeff Lindsay costs at Barnes & Noble?
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