This is a quite a short manga. It feels like an introduction to an anime or just a side story to a novel. Now, I haven't read or seen the novels/anime, but the chapters do seem like fillers with almost no background to the characters and main plot.
The story contains an overused theme in ecchi manga/anime which revolves around a weak perverted male MC that feeds power to the strong female MC through his "pervertedness".
It definitely doesn't lack on the ecchi side(which plays a big part on the story's development) and the supernatural side of it complements just fine.
Character design goes well with the style of manga. The drawings are good and the character designs nice, but nothing out of the ordinary and nothing new by a long shot.
The characters are cliched. The female MC obviously sees the perverted MC as disgusting but as the story progresses her attitude towards him changes, tsundere"-like if I may. She ends up stuck with him and requires his "power" in order to live and fight, which results in her change of mind/heart.
It was hard to think of this as a harem since there is little to no attraction from the other supporting characters towards the main. It mainly revolves around the occurrences between the two main characters and the unlikely ecchi incidents between them and the other supporting characters.
A good manga for ecchi lovers. It's short, goes straight to the point, and could even be defined as "borderline hentai" in some parts. It lacked in story and character development but you will find it enjoyable if you're just looking for a decent ecchi manga.