The Demon King (real name Kyoichi Yasaka) is the main antagonist of the KonoSuba series, and is currently engaged in a war with Belzerg, and by extension the world at large. He resides in his castle, which has a powerful barrier protecting it, even being able to withstand the power of the Crimson Magic Clan. He is rumored to be incredibly powerful and ruthless, however, it seems that many of the negative rumors may be baseless accusations by the inhabitants of Arcanletia, and according to the Prime Minister of Elroad he is a very sophisticated and reasonable person. It is also mentioned by Kazuma that one of the main reasons the Demon King is at war with Belzerg is due to the antagonization by the Crimson Magic Clan and the Axis Order. However, this may possibly be a joke. He has since been defeated by Kazuma using Explosion Magic.
With the existence of a Demon King, every single monster that lives in the world of Konosuba continues to get stronger and stronger as time passes.
He is mentioned to be quite old, looking to be around in his mid-100's with gray hair. He generally has a human appearance, save for the noticeable horns on his head. He wears a pitch-black outfit. When his powers are active, his body would emit a black mist that strengthens his allies.
The Demon King has a personality befitting a final boss. According to Vanir, it is the Demon King's job and expectation to be challenged and eventually defeated by brave adventurers. He apologizes for his subordinates rudeness and attempts to have a traditional and honorable final battle with his opponents. The Demon King initially speak in a formal and haughty manner, but he's much more pragmatic and flexible than such a manner would imply. The Demon King changes and adapts his combat style to suit whoever he's facing and, despite his initial dismissal, acknowledges that Kazuma has proven troublesome. The Demon King doesn't like to be compared to the strangeness of his Generals; he makes it clear that he's trying his best to manage them and that it has been very stressful for him. He's shown to be very caring for his daughter.
The Demon King is very powerful, though he is noted to be past his prime and not necessarily the strongest in his army. One of the main reasons he is Demon King is his ability to command and control the Demon King Generals (who's eccentricities often cause many problems.)
Ally Enhancement: The Demon King's primary ability. Any ally near him gets a massive increase in ability (making even weak creatures comparable to Demon King Generals), though in his old age the range is limited to those in his immediate presence. He is also able to give permanent enhancements to his generals: Beldia, for instance, was given high resistance to Holy Magic.
Durability: He demonstrates resistance to non-enchanted attacks and Eris says he is likely immune to debuffs and instant kill effects. He was able to tank his own Inferno spell without much issue.
Strength: Even in his old age, the Demon King is said to be able to crush an ogre's skull with his bare hands.
Speed: He was able to move faster than Kazuma could see. Even after Kazuma had been buffed twice in speed by Eris and Aqua, the Demon King was still able to keep up with him despite being sealed by Aqua and having an injured leg.
Magic: The Demon King has powerful at his disposal, and had enough mana to keep up with and surpass Kazuma's Manatite.
- Teleport: The user can mark locations and instantly send themselves or other people/objects to those locations.
- Create Earth Golem: Summons a golem out of nearby earth.
- Create Earth: Creates a mound of dirt. The Demon King's version of this spell was able to create enough dirt for a golem that rivaled the size of Kazuma's (which was created using high quality Manatite).
- Cursed Lightning: Summons a piercing bolt of black lightning.
- Inferno: Creates a powerful and large burst of fire.
- Although he has a Japanese name, the Demon King isn't from Japan.