Well, I went into that drama not recognizing the main actors from anything else, so I had no biases in that regard. I did low-key ship Sunny and Wang Shik, and I was sort of (but not very) disappointed that nothing ended up coming of that (likely due to the schedule conflict for Choi Jin Hyuk) - however, I was more interested in the trainwreck that was the relationship between Sunny and Lee Hyuk. It's not because I really wanted them to end up together (although in an alternate universe, where he'd been given a normal upbringing and had been taught basic values/respect, that'd be great... but it'd be a different story.) I just found them more interesting to watch. (So much so I ended up making an MV featuring them actually.)
To answer your question, for most of the show, I thought of Wang Shik as the male lead, since he was presented as more of a protagonist and ally of Sunny's with a common purpose. However, by the end, it definitely seemed that Lee Hyuk was being portrayed as the male lead; rather than a villain, he sort of became an antihero. We also see Sunny mourning the loss of Lee Hyuk. Surprisingly, the same attention is never given to Wang Shik. I see that as an oversight by the writers.
I found Lee Hyuk to be the most entertaining/interesting character and definitely came out of it a fan of Shin Sung Rok. The next drama I watched was Liar Game, and I still intend to find other works of his to watch.