Those who are fans of the Dexter book series know that Deb fell for a character named Quinn and develops a romantic relationship with him. Though there are differences between the Quinn in the book and the Quinn in the show, maybe it's the creators' plan all along to eventually have the two partners become more than friends when the show reaches its endgame. It has been hinted that Quinn may have more than a friendly interest in Deb when he ask Deb if she would want him as a plus one for Dexter and Rita's wedding.
On the other hand, it could be Masuka. Hey, stranger things have happened. As far as the beginning of the series, there has always been this banter between the characters. While Deb may not have been flirting with Masuka beneath that banter, the same can not be entirely said about Masuka. Maybe Masuka had a crush on Deb for a while and maybe the aftermath of Frank Lundy's death is drawing them closer as friends at first. I don't see them getting together by this end of the season but, like I said, probably by the end of the series.
What do you think?