Manga versions of long light novel series are often just disappointing advertisements for the novels. Unlimited Fafnir is a little bit better than that... but just a little bit.
Story: 5/10 There are WAY too many plot elements for 20 chapters. Three years earlier, a boy mysteriously saves his sister from a dragon. He then gets whisked off to be an assassin for the military, while she develops supernatural powers, related to the dragons, and is sent to a school to train those. This series starts with him joining her at that school--the only boy there--and encountering a pretty naked girl.
Then come the harem antics, fights with three different dragons, betrayals at the academy, mysterious backgrounds for each of a dozen characters...
Like I said, way too much plot for 20 chapters. None of it is bad, but it all happens too fast to be comprehensible.
Art: 8/10 The art is solid. The character models are standard, shounen "cute", but done well. The backgrounds and battle sequences are pretty well drawn.
Character 7/10. Too many characters as well. I liked them all, for the most part. They all seemed to have interesting stories, and I wanted to know more about them. But, there is no depth to anyone... cause you know, a dozen characters in 20 chapters.
Enjoyment: 7/10. If you check your brain at the door, there are some amusing harem antics between well-drawn battle sequences. So yeah, pretty fun.
Overall: A generous 7/10. It's not a bad manga, at all, but it's not really good either. If you want to know whether to try out the Light Novel series, it's worth a read. If you really love manga like Trinity Seven or High School DxD... well, this is a not-bad manga in the same genre, and it won't take up much of your time.
For the rest of you, go elsewhere.