The story's protagonist is a self-proclaimed "Bad-ass Freakin' Overlord" named Lord Zetta, who attempts to prevent the destruction of his netherworld prophesized by Pram the Oracle. After discovering the Sacred Tome, Zetta finds a passage claiming that his own stupidity would be the downfall of his Netherworld. Infuriated, he burns the book, only to remember immediately afterwards that destroying the book would destroy his world. He quickly responds by confining his soul to the Sacred Tome and ventures to rewrite his world into existence by having the other Overlords of Netherworlds write wishes within his pages.
Lord Zetta (Seiyū: Takehito Koyasu English Voice: Crispin Freeman) - Lord Zetta is, in his own words, "the most badass freakin' Overlord in the cosmos". Though not too bright, Zetta makes up for his shortcoming with sheer power and attitude. (This is reflected ingame as he has the highest stats among the characters.) Even though he's gone through a major physical change, his personality hasn't really changed. Zetta later appears in Disgaea 2: Cursed Memories and in Disgaea: Afternoon of Darkness as a hidden boss. In the DS remake of Afternoon of Darkness, the PSP remake of Cursed Memories (called Disgaea 2: Dark Hero Days) and the PSP remake of Phantom Brave (called Phantom Brave: The Hermuda Triangle), he's been confirmed as a playable character in his Sacred Tome form. As he has only made appearances outside of his own game in his Sacred Tome form (which is his most recognizable state), it is assumed that in Canon, he either never changed back (which breaks from the ending), or Trenia swapped places with him once more, or this takes place before the ending of Makai Kingdom. The last explanation seems to be the most possible one, as shortly after Zetta is recruited into the player's party in Disgaea DS, a mysterious character (heavily implied to be Pram) talks about losing a 'precious little plaything', which was the same term Pram used for Zetta in the Good Ending before acknowledging her own fault and respecting Zetta's feelings for Salome. However, he has appeared in his original form in Disgaea 4: A Promise Unforgotten as a hidden boss, and after being defeated, joins the party. This marks the first time outside of Makai Kingdom that his original form is playable.
Pram the Oracle (Seiyū: Kaori Mizuhashi) English Voice: Amanda Winn-Lee) - Pram is the youngest Overlord in the cosmos, having managed to take over her father's Netherworld when she was only two. While she stands out for her ability to predict the future and her sheer level of power, it is said that she possesses another rare ability among the Overlords - Shapeshifting - though no one can truly verify this. Rumors say that she has only used it once. She is later referenced in Disgaea 2, appearing as the image for the Omega level Ice spell. She is the one who rewrote the tome and which caused the events of the game to unfold, though she later atones for what she does and helps Zetta in an attempt to bring Salome back but is cut down by Seedle. She apparently managed to survive somehow, as she was seen again in Zetta's castle along with Salome, Alex and Zetta at the end of the Good Ending, questioning whether the mysterious 'The One' was a fake. It is also revealed that she can't actually predict the future, but reads the Sacred Tome. She appears as a downloadable character in Disgaea 2: Dark Hero Days, Disgaea 3: Absence of Justice and Disgaea 4: A Promise Unforgotten, and as a downloadable boss in Prinny 2: Dawn of Operation Panties, Dood!.
Dark Lord Valvoga - Dark Lord Valvoga is actually three Overlords in one, consisting of Micky the Demon General, Ophelia the Fallen Angel, and Dryzen the Star Dragon. Valvoga is often summoned to be the "last boss" in various video games. Micky forms the upper body, Dryzen forms the lower body in the form of a gigantic dragon skull with a glowing red orb within its mouth, and Ophelia forms herself into a head in a hole in Dryzen's skull, placing herself directly in the center of the "body". Valvoga as a whole serves as the image for the Tera Star Spell in Disgaea 2.
Micky (Seiyū: Tesshō Genda) - While Micky is technically the one in charge of the body, he is often manipulated by other Overlords, Dryzen and Ophelia in particular, because of his timidness. When he finally learns to stand up for himself and make his own decisions, his true form is unleashed, in a more compact, humanoid form.
Ophelia (Seiyū: Miki Narahashi) - Ophelia tends to be the "brains" behind any operation Valvoga does, and tends to be rather subtle in her manipulations. Whenever someone else tries to manipulate or pressure Micky, Ophelia steps in, though she is primarily doing it out of self-interest, as no one controls Valvolga but those who are part of Valvolga. At the end of the game she is revealed (in a very humorous scene at the end) to be the mysterious force in the game known as "The One", but of course nobody else in the game knows this. Dryzen (Seiyū: Takashi Nagasako) - Dryzen bullies around Micky because it's fun, as well as when it benefits himself. Dryzen is massive and insane, with his most common statement tending to be little more than "Kill!". Alexander the God of Destruction (Seiyū: Kenji Nojima) - Alexander is Zetta's chief rival. As Overlords constantly wish to expand, many Overlords attempt to conquer other Netherworlds, and Alex is no exception. Alex has attempted to take over Zetta's Netherworld hundreds of times, and employed over a thousand assassins, but none have yet succeeded. While Zetta considers this proof that he won, Alex claims that the fact that Zetta hasn't taken over his Netherworld is proof that the battle's been a draw, and that he can still kill off Zetta at any time. Even with Zetta being trapped in a book and his Netherworld destroyed, Alex continues to try and kill Zetta, in an attempt to claim the title of Strongest Overlord. He appears greatly upset when Zetta is cursed by "The One" and attempts to kill him before the curse does, but is stopped by Salome before he can go through with it. He appears later on in the game to see why Salome interrupted their fight but after hearing Seedle's story becomes disgusted with the Overlords and battles him himself while Zetta confronts Salome. Zetta refers to him as an "Asshat". Alexander later appears in Disgaea 2: Cursed Memories as a cameo appearance, appearing as the image of the Omega Level Wind Spell (as Lightning, his preferred element, is not present in Disgaea games). According to artwork, he has a little sister that has yet to be named.
Lady Salome (Seiyū: Ai Orikasa English Voice: Mary Elizabeth McGlynn) - Salome is Zetta's former apprentice who gave up her humanity to live in the Netherworld. Despite having immense power, she has an unnaturally small amount of Mana power, which creatures require to survive in the Netherworld, leaving her body to die slowly. However, she intends to keep this a secret from everyone - especially Lord Zetta. She fell in love with Zetta and vice versa; her wish was to only die in his hands. It turns out the reason why she had a low amount of mana was because, little by little, she kept transferring it all to Zetta which in turn made him the Overlord he was.
King Drake the Third (Seiyū: Norio Wakamoto) - King Drake the Third is a lion Overlord who considers himself to be the most charismatic of the Overlords, though most of the other Overlords just consider him an idiot. Despite his claims of being a great hero, Drake has no compunction with manipulating things to his own advantage, even taking advantage of an enemy when they're sleeping. He is, however, still powerful, as only powerful overlords are able to exist in the same space that the other Overlords spend time in. Drake is the game's 'silly character', even to the point that if you are to unlock him, his proficient weapons are odd, such as Pie. He appears as a downloadable character in Disgaea 3: Absence of Justice and is referenced by Pram in her downloadable event in Disgaea 2: Dark Hero Days.
Demon Overlord Seedle (Seiyū: Shūichi Ikeda) - Demon Overlord Seedle is the Overlord of the Underworld (Making him the "Underlord"). Though he was once an honorable samurai, said to be a part of a party of Braves (a reference to Phantom Brave), the circumstances of his death left him filled with an unnatural fury, allowing him to force his way into becoming Overlord. In addition, he seems to hold an unknown link to Salome. Late in the game it is revealed that while he was still alive he was followed by a still alive Salome who he had become obsessed with. One night during their journey he forced himself on Salome but she stabbed him to death and fled. It is because of this that Seedle holds such hatred for Salome as being killed by a woman apparently left a large wound on Seedle's ego. He was apparently killed by Alex during the end of the game.
Dragon Overlord Babylon (Seiyū: Takeshi Aono) - The massive Dragon Overlord Babylon is the oldest Overlord in the cosmos and once ruled over it all. His wisdom and size gave him an advantage over almost everyone. He used to have near limitless mana power when he was younger, but he lost it all after he was cursed by a mysterious being known only as "The One". The strain of resisting the curse left him battered and a shadow of his former self. From there, he slowly wasted away over millennia, eventually becoming senile, forgetful, and half-blind. He does command some degree of respect from the other Overlords, even the egotistical Zetta. He has the ability to transform into a humanoid form, of which is usable by the player. In an extra map, Zetta can time travel back to when Babylon was in power, and can defeat him, earning his humanoid form. By this, it is revealed that it is Zetta himself who defeated, and took much of his power, making him 'The One', though this contrasts Ophelia's statement of her being the same being. It is hinted that Babylon's power loss was due to the psychological effects of him losing to an unnamed opponent (as he did not know who Zetta was when the latter travelled back in time to confront him, furthermore, he was already obsessing over The One before fighting Zetta), and that he never really lost his power, but rather became depressed after his defeat and allowed his powers to diminish.
New Characters Petta - A new character in the upcoming PSP remake of Makai Kingdom, as well as a downloadable character in Disgaea 4: A Promise Unforgotten, Petta is revealed to be the daughter of Lord Zetta. Her origin and significance have yet to be explained, though she does bear a strong resemblance to Zetta and is able to use the "Petta Beam", her own variant of her father's "Zetta Beam". She features in her own scenario in the remake.
The gameplay is somewhat similar to the gameplay found in Phantom Brave and Disgaea: Hour of Darkness, other games created by Nippon Ichi. The characters summoned in battle have freedom to move within a circle set by the characters' movement points. Victory is achieved by scoring a set amount of points, which are given by interacting with items on the map, discovering extension maps, and eliminating enemies. Several of the classes from Disgaea: Hour of Darkness exist in this game as well, such as Warrior and Mage, but some of the equivalents of previous classes are different. Some classes also have distinctions between sexes.
There are a variety of weapon types, each with their own expansive sets. Characters can equip armor and various other accessories to aid in combat. Each class has four weapon types in which they are proficient, and can learn new moves to use in combat as characters master the weapons.
Vehicles are a new addition from previous Nippon Ichi games. They are similar to moving, battle-capable buildings. They vary in size, shape, and purpose, but are generally faster than travelling on foot. Vehicles can damage opponents and can level-up. However, levelling a vehicle is different from levelling a character. Leveling up high ranked vehicles is difficult, and in actuality, impossible to raise them to 9999, as the cost will effectively be more than the player can ever make.
Many of maps are randomly generated. While, in many cases, the first map in a storyline mission is pre-set, the "extensions" to the map are often randomly generated. The randomly generated maps creates the potential to have vastly different situations even when replaying the same map. Within the game, there are two separate types of extensions: "Locked" and "Secret". While both extensions can be revealed by throwing something out of bounds into the extension, "Locked" extensions can also be unlocked by destroying a specific unit on the field, which is marked with an icon saying "Key".
Once an extension is revealed, there may occasionally be "special events" which occur as soon as the extension is unlocked. While many special events simply affect the enemies that appear in the extension, others will paralyze everyone on the battlefield, put everyone to sleep, or other effects.
The game's characters hail from alternate universes, which Pram describes very clearly at the beginning of the game:
"There is, of course, more than one universe. In one such world, in the corner of a backwater galaxy, humans rule over a senseless planet. However, that's about as important as a speck of dust in this pluralistic cosmos. Here in the Netherworld, things work the same way."
Even though many of the characters come from different worlds, all of the universes and their characters exist within the same story continuity.
Makai Kingdom's multiverse helps expand on some of the elements of alternate dimensions mentioned in Nippon Ichi's other games centered around demonic Overlords, Disgaea. Every universe has its own Netherworld, therefore many Overlords exist in this continuity to vie for power.
In Makai Kingdom, characters from previous Nippon Ichi games (Disgaea and Phantom Brave) may make appearances as challengers to Lord Zetta. The characters from Disgaea, i.e. Laharl, Etna and Flonne, are treated basically the same as the other Overlord story-characters - they hail from another Netherworld and are seeking power, revenge, or 'love and justice'. The characters who arrive in Zetta's Netherworld from the Phantom Brave world are transported there by a powerful being from the Phantom Brave story.