First of all, it has the human weapon as male (a rather good looking one at that) rather than female as seems to be the standard in anime, seemingly push this from an ecchi shounen aspect to a more shoujo one. Most series if they want to reverse it, either make the human weapon as some ugly robot, as some monster hybrid or some female cute monster. That's what attracted me to this series in the first place. I was wondering where they would go with this.
After getting hooked to Melan and wanting to see more of him, it's then that the story hits. It really doesn't pigeonhole itself into the "Proxy battle", "Mecha" affair that it seems like it's going to shove itself into. The mystery deepens when we find out that Melan is protecting Marin for a reason. You want to find out why. Toss in a magical transformation device and some romance and you really feel for the characters. It's stops being about a formulaic trope that so many anime base themselves on.
Others have mentioned the ecchi aspect, but really the way it's done in this series more or lets blends it away. Yes you see Marin's panties, but she's underage and they don't really point to it and say "HEY LOOK!" like every other ecchi and non-ecchi anime does and make it the focus of the anime. You find out that the reason you can see them in the first place, is probably because she's poor and doesn't have any money to buy new clothes. Even the scenes of nudity is for a reason and they don't parade it down the street like it's the point of the episode. People are simply nude because they have no clothes, or they can't wear them at the time.
Really over all, the story is VERY original. It doesn't have an aspect you can plop into a category. Similar to DT Eightron in that respect. The ones who came up with the story actually sat down and probably did some world building versus sit down and go "Yeah let's make another anime about a harem... with ecchi. Yeah, like those other five hundred out there." It's really want I want to start seeing in anime. More original stories that don't ruin the experience by having ecchi parts all over the place (Umi Neko and a few other mecha shows), toss in a harem aspect for no good reason (yeah, let's have a bunch of women... for no real reason. If anyone asks... we'll say it's SCIENCE!), or simply be on long string of colosseum battles (YuYu Hakusho, Shaman King, Yu-Gi-Oh, Bleach, the list goes on). It even averts the tried and troped "Let's have a guy... that controls a woman... but she's really a sword/demon/mecha/killing doll/dingo/familiar!" It really doesn't want to make me watch another master-slave relationship being played out on TV where the master is male and the slave is female. I see that in real life too much thanks.
There are anime where the master is female of course, but they add so much ecchi to it that is shoves it back into the shounen, ecchi, boin, harem category. This averts it by making the master female and a child and the slave a good looking bio-mecha type where you don't see the romance coming until the characters start to get closer to each other. It's played more realistically.
Overall this anime is good, because it doesn't try to be like every other anime out there. It makes it's own path in the world and doesn't rely on tropes and cliche out there to get it done.
