To begin, this review is spoiler-free, so if you've watched the anime and are contemplating reading the light novels you don't need to worry about being spoiled with my review.
Simply put, Highschool DxD is an amazing light novel. If you enjoyed the anime at all, I can absolutely guarantee that you will love this. With season 3 approaching, the anime has literally only begun to cover where the series really starts to get good. And from that point on, the fantastic ride that is DxD only gets better. And better. And better. Hell, volume 19 will be released in 2 months and it's STILL getting
The first 4 volumes of the story are basically set-up for the rest of the series. Volumes 5-6 are where everything starts to really pick up. Now, that doesn't mean volumes 1-4 are bad; not at all, just look at seasons 1 and 2. In fact, even if you've already watched the anime, I recommend reading the volumes they covered; it provides more detail and depth than the anime could show.
But volume 5 onwards is where, put bluntly, shit gets real. It gets really intense(that's actually quite an understatement), there's a lot more romantic development, lots of new characters are introduced, and everything just gets all-around more epic and awesome with each new volume you read. There's also quite a few hilarious situations in the midst of all the epicness, which show that Issei truly deserves the nickname he was given. And yes, I am trying to be as vague as possible as to not spoil anything for you guys. I could seriously write a ton of detailed comments if I included spoilers.
At this point you might be wondering "what about the ecchi?!" DxD's light novels are actually far more erotic starting, again, with volumes 5 and 6. (I apologize if I'm making the wait for season 3 even more painful, I'm just being honest!!) In fact, I'll be completely honest and say it becomes the most erotic thus far starting with volume 15.
Let me explain something: Ever since I started reading DxD, I fell in love with Miyama-Zero's art. Every colored illustration in every new volume (I started reading DxD when volume 16 came out), cover included, has left me speechless, and every non-colored illustration has me praising DxD's illustrator as well. As soon as a new volume is released, one of my first thoughts is "have the illustrations been leaked yet?!" Miyama-Zero-sama's art is just that good, especially in the more recent volumes, where I wouldn't hesitate to call it masterful.
I'll divide this section into 2 parts, Protagonists and Antagonists. But if I were to summarize both sections in one word, it would be Badass.
Protagonists: To start with, let me just say that Ise's harem is handled in such a wonderful way. There's no crazy tsundere beatings or normal harem antics, just friendly competition amongst the girls for their spot in the harem. Not to mention Ise's harem is hands-down the most beautiful harem I've ever seen. All the girls are just perfect. Every single one of them is so unique and they all play a huge role of importance in the series. Some of the girls of Sitri group also get more screentime as well. I can't say more than that or I'd spoil a lot of new characters! Onto the guys, the deep bond Issei and Kiba have is really great. They really are best friends, although Kiba does seem to like Issei a little too much... Azazel is a fantastic addition to the team, watching out for them and helping them with their training, among several other important roles in the series I'll let you discover for yourself. Gasper continues to work on becoming more manly, and Saji actually takes a pretty big role in the series later on. Oh, and Vali's as crazy as ever. Again, I can't say more or I'd spoil certain events and a lot of new characters!
Now let's go into a little more detail about Issei. His growth is astounding. Spoilers withheld, he goes from being an overly-perverted MC to a perverse, respected, beloved, mature and powerful MC who gives his all to protect his comrades and truly cares about all of them. The best part is the execution of this character growth, which you'll need to read yourself. The dynamic between Issei and Kiba is fantastic as well; Kiba takes up Ise's reckless desire to protect his comrades and becomes a powerful swordsman worthy of being Rias Gremory's Knight.
Antagonists: I love DxD's antagonists, in particular the ones introduced in volumes 9 and 16. They're just as diverse as the protagonists and several of them are antagonists you just love to hate (especially one particular antagonist in volume 17). I really can't say more without spoiling their characters, just that they're great.
Everyone, protagonist and antagonist alike, gets the perfect amount of character development and exposition in each volume, it amazes me how well Ishibumi-sama handles it.
One thing I especially love about DxD is its unpredictability; you absolutely cannot predict what's going to happen next. It's hilarious, it's unpredictable, it's extremely intense, all the characters are fantastic, the ecchi and romance are wonderful(another severe understatement!), the art is excellent, overall it's a supremely fun series.
Let me conclude by saying that Highschool DxD is in no way your ordinary ecchi series. The character development for everyone is astoundingly brilliant, the plot is absolutely amazing, and, being vague once again, DxD does not suffer from a certain traditional harem trope that never ends, not to mention there is an actual reason for why Issei acts the way he does up until a certain volume, and it's handled brilliantly. Overall I give it an extremely solid 10/10 and urge those of you who enjoyed the anime to read it - you won't regret it!