Second Lieutenant Mononobe Yuu has been sent to Midgard, to attend the all-girl's school where his sister Mitsuki is the student council president. The school is specifically for people who have "D" powers, who are needed to fight dragons, and both Mitsuki and Yuu fit the criteria. On his arrival, he meets and befriends another classmate named Iris Freya, who is having difficulties with her "D"-related schoolwork. They will do their best to succeed in this dangerous world.
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.