Diabolik Lovers episode 6.5 is a 15 minute recap of the (lack of) story so far and includes every last bite!
There are tons of anime out there that could have done with a recap episode but Diabolik Lovers was most certainly not one of them.
For a start, episodes are only 15 minutes long and the series overall is just 12 episodes. Anyone who has seen the first 6 episodes of Diabolik Lovers would know that nothing worth recapping occurs.
This recap is just an excuse to create a montage of the bland protagonist Yui being bitten by the sadistic Sakamaki brothers. It's basically the so called 'best bits' aka the disturbing bits of the first 6 episodes. I would not say it does a great job at recapping but it isn't terrible as it is coherent.
However, if you have just started Diabolik Lovers and want to drop it but are one of those people who refuse to drop an anime and will always see it through to the end then watch this and then the last 6 episodes. You will be saving time that is not worth wasting on a mediocre anime like Diabolik Lovers. 5/10