I'm writing this under the assumption that readers have seen Season 1 or at least read the manga.
~ Story
The story continues from where Season 1 left off. Keima is still capturing the hearts of girls in order to fill the void in their heart and catch the demons that have taken refuge.
If S2 opening is anything like S1 opening, then it seems that this season revolves around Chihiro, Jun Nagase, and Kusunoki. Also Haqua joins the team