I think the characters from Season to Season are intended to build upon failed relationships that ultimately lead Dexter to self-discovery.
-Debra is his sister and he cares about what she thinks of him, but he can not be honest with her because of that fact. He had Brian come back into his life and he wanted to bond with but couldn't once he saw how clearly the code divided them. Then, he had Miguel who he deperately wanted to fill that role. They got closer to what Dexter wanted from a sibling than he could with Brian but they were both lying to each other and ultimately suffered the same fate as Brian.
-During last season, near the end where Dexter and Rita had that conversation in the car, I think that was the first real step to Dexter feeling at ease with Rita and the possibility of her eventually accepting who he really is. Of course things ended for Rita before that could happen and now we have Lumen, who is damaged just like Rita was, but she knows he kills. I think this is why he is so open at revealing himself to her, because it's almost like closure for his issues with Rita.
-Now we have Quinn, who's basically taking over where Doakes left off. He suspects Dexter and is hot on his trail, but the difference here is that emotions are involved due to his relationship with Debra. Also, we see Quinn doing some shady/illegal things....the real question is, with all the things involved, if Quinn found out who Dexter really was, would he turn him in? Would Quinn see the vigilante justice as needed and turn a blind eye? would he be a "good cop" afterall and take Dexter down/arrest him? or would it be his greed and ego that takes over and he try to blackmail Dexter or simply look at busting Dexter as a way to further his career ?