Now that the series is over I feel all right about exposing Ryuuga. Sort of. We only got to "know" him in the movie. Of course he skimmed through the 13 Rider Speical. He has a great deal in common with Yui and Shiro Kanzaki. When I first looked at his pictures and saw that he was merely Ryuuki writ black, I was very annoyed. Of all the animals they have monsters for, why the same dragon? In the end, though, it makes complete and total sense. What did he want? He wanted to be a real boy, hahah. He had no qualms whatsoever about killing anyone who was in his way.
Since the movie runs with a different history than the series, it's pretty much that he just never existed in the series.
Ryuuga was Shinji Kido's Mirror World self, possibly created by seven-year old Yui.