1) Rita. His wife. This didn't end well. She died at the hands of Dexter's rival, the Trinity killer. She never knew about Dexter's "other life"... unless Trinity, in an ultimate act of cruelty, told her at the end.
2) Lila. His psycho British girlfriend. Also didn't end well. She shouldn't have messed with Dex.
3) Lumen. His girlfriend/partner-in-vengeance. Not a psycho, and maybe Dexter's best actual relationship, because she knew who he was and accepted it, even though she herself was normal. She escaped... but will she return?
4) Hannah. His latest girlfriend/partner-in-serial killing. Different from Lumen, because she too was a serial killer and a psycho. But way, way up there on the hotness scale.
5) Debra Morgan. And, of course, his foul-mouthed half-sister, who appears to have always loved him, but now hates him because he has corrupted her. She found out about his murders by catching him in the act, then helped him cover it up and, ultimately, committed murder herself to save him. Now she's tortured by what she has done. Will Deb end up confessing and turning Dexter in? Will she take matters into her own hands and kill him? What a great character played to the hilt by Jennifer Carpenter.