If you haven't watched Sankarea yet, definitely watch the first OVA, (Episode 00), before you do. Being the prequel, it'll answer some questions you have. Episode 00 is about 17 months earlier. Chihiro is telling a story, about memories, mostly in his perspective but also some is in Rea's. You see some more about Rea's odd relationship with her father and her mother. Chihiro goes on a holiday with his family and Ranko tags along. So you learn more about their history, too. The second OVA, Episode 14, is best to watch after the series, as that's it's proper place. A weird girl appears under thetemple and strange things start happening. Chihiro shows he's smarter than he looks, and it all comes together in the end.