u/Careless_Pumpkin3858

We know from Serena in V16 that there is a fountain connected to hell that practically gives infinite mana and there's an archangel (or were they a normal angel? I don't really remember) that was connected to it and what does the DK do with him?

He just keeps in the castle to fend off the occasional crimson demons attack… DK, you decide to use the guy with effectively INFINITE MANA for f***ing DEFENSE! While already having a barrier that —despite being supported by only 2 generals at this point— can tank more than a dozen EXPLOSIONS. Don't you think having both dedicated to defense is a bit overkill?

Even assuming that the fountain has limited range, the DK could have still used the archangel for offense. All he needs to was:

1. Have the archangel learn explosion from Wolbach if he didn't already learn it, I assume he already learn it since the explosion's only draw back is its massive mana cost which is irrelevant due to the fountain.

2. Have him mark the castle and whatever location the DK wants destroyed with teleport, so whenever he runs out of mana, he can just teleport back to the fountain and recharge his mana and teleport back to the battlefield.

Now, you have a upgrade version of Wolbach's strategy, instead of one explosion per day it would be multiple ones which would easily overwhelm any military base&army, even the stronghold we see in V9 would crumble in like… 4 (5 if generous) back-to-back explosions.

If the above isn't possible for some reason, then the DK could just do what Kazuma did in V17 and give Wolbach a bag of high purity mana crystals and the war would be done in two days MAX, after all, if Kazuma could afford it then a king with the an entire military budget should definitely be able to afford at LEAST a few dozens of mana crystals which would be more than enough to destroy both the stronghold and the capital.

For someone that even Kazuma describes as intelligent, I'm surprised that the DK didn't use such an obvious and effective strategy that would certainly guarantee victory.

The DK has been built up ever since the beginning of the series, he's easily one of the most important characters in the plot.

One of the most things I personally anticipated entering V17 was how Natsume was gonna handle the DK, after all, considering how they fleshed out the DK generals, I was looking up to what kind of unique and funny twist Natsume saved for the DK, I thought that they would do something cool with him. Guess how Natsume handle the DK's character. . .

IN THE LAMEST WAY POSSIBLE!!

1. Not even a SINGLE illustration of him!
2. Appears in only two chapters with barely ANY dialogue and all of it is with Kazuma, we don't see him interact with any other character. Not even his daughter and generals!
3. The bar minimum characterisation is given to him, which boils down to "being somewhat similar to Kazuma", even Hans is more fleshed out than that!

I still don't understand why Natsume made the main antagonist of Konosuba —a character-driven story— barely a character! Like, was the entire point of the DK to drop multiple hints at him from others characters (Ragkraft, Serena and Vanir) only to not follow up on ANY of it?!! Or what, they thought that we would find the DK boring and uninteresting and whatever filler BS that was in V12&13 NOT??

Now that I have aired my criticism. Let me suggest some ideas to fix this.

Know the minimal characterisation we were given? Let's expand on it! The DK could be similar to Kazuma but also the inverse to make the parallels between them clear while making the two distinct characters from each other, for example:

The DK could have a character arc opposite to Kazuma's; from loving this wonderful world and genuinely want to do good into growing slowly to hating it with each time the world screws him and his loved ones over.

One other thing that could help is introducing him earlier in the story either by him showing up or through flashbacks. V9 feels like the perfect opportunity to do this or if that requires to change then we can just delete V12&13 and have something that focuses on the DK in there place.

This could also help the issue of the comedy getting repetitive in the latter half of the series (looking at you Darkness) by introducing some variety, as in the shenanigans that happen on the DK's side. Or even the vessel to deliver some of Konosuba's most emotional moments.

TL;DR: the DK had so much potential to add a lot to the story but Natsume thwarted it all for some reason.