For those who have watched the actual show for this series (which you should do before you watch this OVA, or you may end up somewhat confused), I'm glad Theo was the main character of this OVA. I say this because of the lack of real character development he had in the main show, he was always just... there; as the main brain/researcher of the team, which is great and all, but it left very little to love about his character (other than the fact that he's comedically pretty). In this OVA, we get to see his main love for Anothers, and where his lovefor them possibly stemmed from. We get to see so much more emotion out of him compared to the main show, and when I say emotion, I mean EMOTION. Really, this OVA WILL make you cry, and will make you absolutely fall in love with Theo's character.
I guess my only complaint would be though, that there isn't enough Kohaku, at all. Sorry, I needed to see my pretty boy.
I also loved that the OVA was centered around vampires, a much appreciated surprise after finishing the main show. Although, I'd like to complain and ask why this OVA isn't tagged with the 'vampires' genre? It literally has vampire in the name...
Anyway, watch the main show! It's only twelve episodes long and is a pretty chill and well paced anime to watch! You'll have to get passed the clearly budgeted art style, but it's really a good show otherwise!