SPOILERS AHEAD
Overlord Zetta, the most Badass, freakin' Overlord.
Overlord Zetta is the protagonist of Makai Kingdom: Chronicles of the Sacred Tome. He is the strongest Overlord in the Cosmos, and isn't afraid to show it. He receives a prophecy from Pram the Oracle telling him that his Netherworld will be destroyed. He rushes to the Library of the Damned in an attempt to retrieve the Sacred Time, which Pram says is the cause of his loss. However, he is impeded by Raiden, Alexander's right-hand. Zetta defeats him,only for the Library, which is part of his Netherworld apparently, starts to crumble. In an attempt to save himself from the destruction, he Confines himself into the Sacred Tome, becoming it. Now he must rely on other Overlords to create new Netherworlds for him in order to conquer them and repair his empire, hopefully regaining his body in the process. Whether he gets his way depends on the ending he receives. In some, he is is killed off before he can get his body, in others, he performs a stupid action which jeoradizes his body or himself, and the canon ending, he gains his body back, the curse of The One being a fake in the end.
SPOILERS END HERE
Personality
Zetta is everything a demon usually is: selfish, arrogant, self-serving, violent, greedy. However, he isn't completely heartless, as hinted by his romantic involvements with Lady Salome. He is very short-tempered, often quick to anger. He is also very dim-witted, rushing into battle headfirst, and failing to see through plots and abmornalities, making him very easily manipulated. He seems to be unaware as to others' views of him, which is the reason Salome left him before the game's main storyline.
His battle aura color is purple, flashing with dark and light purple shades/shockwaves when showing his Mana Power. When desperate, it flahses fast, the wave of energy flowing upwards past him. When it is being sucked away or fading, it flashes periodically in shortened bursts.
In battle
Although Zetta is currently the Sacred Tome, he once was a very powerful Overlord. However, since he has no limbs, he cannot fight, therefore having to send in fighting units to do his battle for him. (Which is completely dependent on the player's preferences) Although, you breifly control his human form for the first tutorial map, it can be recruited later in the 2nd Cycle or higher.
Recruitment
Although Zetta is the Tome, the player can recruit his Human form by writing the wish "I wish to fight the strongest Demon!" for 30,000 Mana upon reaching Level 300. Zetta encounters his original body, it coming to life in it's own. Trenia warns Zetta that Overlord Baal is within Zetta's body, possessing it. This forces confrontation. Upon defeating his body, Baal escapes, leaving the body. Zetta then decides to take the body back with him, despite viewing it as morbid. Be sure that Laharl is not the Leader when choosing this Stage at the Makai Gate, otherwise he will destroy Zetta's body upon winning the battle, forcing a new cycle. The body has 160% Aptitude in all Statistics, but can only use ATK-based weaponry. It is good with Swords, Daggers, Beam Swords, and Katanas. It's unique skill "Omega Drive" is a single-target skill, doing monstrous damage.
Trivia
Zetta appears in many of the newer NIS games, such as Disgaea: Afternoon of Darkness and it's alternate installment Disgaea DS. In Disgaea DS, he can be recruited into battle upon defeating him. He lacks battle voices and can barely move at all, but makes up for it with 180% in all Aptitudes. His two skills "Zetta Beam" and "Badass Overdrive" being a shout-out to Makai Kingdom.
He can also be recruited in Disgaea 2: Dark Hero Days and Disgaea 3: Absense of Justice. He is recruited by passing bills for recruitment in the Dark and Classroom Assemblies. Upon Magichanging with a character, they can use the skill "Invite," which is a reference to the fact that inviting a facility onto an ally or enemy in Makai Kingdom can damage them to a small degree.