If you love flashbacks, then this series is for you!
The beginning of the manga is fairly episodal (and if I may say, overly sentimental): a soul gets wrongly sent to hell, Kucabara finds the "innocence item" (the proof the soul had actually led a good life and was unjustly damned), and the innocent soul gets revived. We get 4-5 storylines like this before the larger plot kicks in, namely the real reason Kucabara was exiled from hell. And this is where the series goes flashback heavy and starts alternating between events in the present and stuff that happened in Kucabara's past. The series decides that it wants to tell us, all at once, apparently, almost everything there is to know about Kucabara. The problem with this is two-fold: firstly, flashbacks are a drag, and secondly, the flashbacks are, unfortunately, far, FAR more interesting than anything going on in the present.
The present storyline is, in my opinion, mostly silly and uninteresting. We get all the shonen manga staples--the training sequences, the battles with lesser villains building up to a showdown with the major villain, etc., etc., but the problem is, all of that is done in a somewhat lame way: For example, in order to build up their teamwork ability and train to go back to the demon world, Ida Maria and Kucabara ...play tennis. A lot of it feels like the series is going through the motions: a training sequence and various obstacles are expected, so here you go--never mind the fact that the guy Kucabara is fighting doesn't actually have anything to do with the major villain.
Ida Maria, the series' major female character, pretty much symbolizes all of Defense Devil's problems to me: she is a fairly undeveloped character, acts difficult for the sake of acting difficult, but we're expected to ignore all that because oh look! here's fanservice! (Guys, she gets naked or partially naked all the time, what more could you want?) But don't worry ladies! There's a male character in the series who is just as undeveloped as Ida Maria, but who also likes to take the random shower or two! Whatever your preference, these mangakas provide! And this is what this series comes down to: a whole lot of flash (or is that flesh?) but not much substance.
I don't really get it; had this series been the one we see in the flashabacks, it could've been great: the story of a kindhearted demon prince who rises and falls from power. Instead the present-day stuff feels like we're reading the conclusion to a great story, and not the story itself. I'd have dropped this manga long ago if it weren't for the fact that I've loved their other manhwas and that I keep hoping this manga will settle down and start living up to its potential at some point. Hopefully I'll eventually come back and edit this review when the series becomes great so that I can say "it had a rough start but eventually got better," but for now, I honestly can't recommend Defense Devil.