It's a 13th OVA episode that's missing from my Another DVD set. This seems odd. It's better than a lot of OVA episodes and it came out only a couple of months after the TV series. (The latter ran from January to March 2012, then the OVA was released in May.)
I suppose one wrinkle is that people might try to watch it out of order. It's a flashback episode, set before the TV series and leading into ep.1, but it contains spoilers and you'd be better off not watching it until after you've finished the show. Ideally at least a few weeks afterwards, in fact. The TV show is a horror series that ends with an apocalypse, as far as the characters are concerned, but this OVA is a gentle piece about the relationship between two girls. I quite like it, but you'll be doing it no favours if you treat it as ep.13 and just plough on with it straight after watching the series.
It's fairly inconsequential, despite a death. It's basically a glimpse into the kind of person Mei Misaki used to be, in the days when she was responsive and had emotions. We see her with her cousin. The do girly things. They go shopping and visit an amusement park. They mess with people's heads by dressing identically and wearing matching eye patches, although admittedly the latter is for good reasons.
This is also nice because we never saw Mei's cousin in the anime. She's dead. Mei was visiting her corpse in ep.1. So, yes, we know what's going to happen and this OVA episode is playing with that foreknowledge.
This episode isn't a big deal. If it had been, they'd have put it on the English-language DVDs. It's a low-key slice-of-life affair in the first half, before the second half starts ambling towards for what we know will happen. I like what it does for Mei, though. I'd never really thought much about her and her cousin, but here we see them together. It's a bit of an eye-opener. Fujioka's brighter and more cheerful than Mei, so the two of them hanging out together is showing us a Mei we've never seen before. I'm glad I watched it.