Dezember Zwolf (ディツェンバー・ツヴォルフ Ditsenbaa Tsuvorufu) is one of the 12 Month Demons and one of the 4 Great Demons of the Demon World. He is the main antagonist of Senyū for the anime and Chapter 1. He is a secondary antagonist onwards.
He wears a shadow on his face, which he can remove and which often changes shape and dimensions. Dezember is tall and human in appearance, side from the shadow. His eyes are black and red, with the iris/pupil taking the shape of a die where the number displayed on the die reflects the core/number that the body belongs to. His hair is black and slicked back rather poorly, with two long antennae that fall forward.
Dezember's clothing consists of a white t-shirt and jeans under a large fur-lines black coat. He wears boots that sometimes have decorative buckles or chains, and sometimes aren't boots because Haruhara isn't consistent. His ears are pierced and he wears die-shaped earrings, two on the left ear.
Dezember himself states that he "hates lies and justice".
He is a decidedly dramatic individual who takes his pride and his role as a 'villain' very seriously and very theatrically. Dezember has extremely poor control of his emotions, though it's shown in Book 1 that he's aware of this and trying (unsuccessfully) to improve in this respect.
Dezember is extremely good at planning, as shown by his ability to stage two successful coups (one in the demon world, by killing Rchimedes II and one on the human world by replacing the king's advisor with Mai Fünf), but he tends to get carried away by excitement in the heat of a moment. The plot of Book 1 and part of Book 2 come about mainly because of Dezember's planning.
He is extremely devoted to Rchimedes 1, later shown to be because Rchimedes created him and Dezember wants to repay that and "make the Demon King happy". Because of this devotion, he's rather easy to manipulate or catch off guard with the mention or appearance of Rchimedes, something which Elf takes advantage of in Book 4. Dezember is elated to simply be anywhere near Rchimedes or remotely acknowledged by him.
Dezember, to continue his dice theme, is made of six separate 'cores', which are six individuals formed from facets of the full Dezember's personality, and they can separate or re-combine at will. When all six are in one body, they generally take on the personality of One, the main Dezember, with an added measure of cockiness due to the increase in power. One is the main core, and generally serious and in control of the others and his 'team'. Two, Four, and Five, are shown the most aside from One. Two is peppy and generally shown smiling, and generally indistinguishable from One in appearance. Four has white hair and is extremely antagonistic. Five has shorter hair than the main body and tends to panic and frequently cries under stress.
When all 6 cores are combined in one body, Dezember has six eyes, with each eye having a pupil of a different face of a die.
Most of the plot of Senyuu is due inadvertently to Dezember Zwolf, who sets off on a quest to revive Rchimedes 1 from his liminal dimension prison and ideally kill the hero Creasion in the process. To achieve this, before the events of Book 1 (shown largely in flashback), Dezember creates a team of demons-- Mai Funf, Februar Zwei, August Acht, Neun September, and Juli Sieben-- that are to help him stage a coup. Using Neun and paying off Elf November, Dezember captures Rchimedes 2, then creates a dimensional rift to send demons into the human world. Mai Funf is set as the King's aide and Dezember a secret advisor, telling the king he should send out heroes to search for the Demon King.
Dezember attacks Alba and Co., at which point he discovers that Ross is Creasion and attempts, ultimately unsuccessfully, to kill the two. After these failures, he goes back to the Demon World when the portals between the two close.
When Alba goes to the Demon World to find Ross, Dezember is shown to be still searching for Rchimedes 1 there. He is also revealed to now be healthy enough to combine all of his cores into one main body. He meets up with the team of heroes after the demon world tournament and absolutely obliterates Janua Ein and Ross in a fight, revealing that he is essentially invulnerable when all six cores are in the same body. He tries to team up with Rchimedes, who disappears when Guilty Justice arrives. Justice declares that Dezember is a far prettier darling than Alba and immediately clings to Dezember, who is sincerely demotivated by all this nonsense and disappears as well.
During the final battle with Rchimedes, he appears and Rchimedes' assistant, which Rchimedes only appreciates when Dezember shuts up. When the battle gets difficult, Dezember essentially kills himself for Rchimedes, declaring "This is my love," before giving all of his mana to Rchimedes, which reverts Dezember back into inanimate dice.
After the battle, the dice are reclaimed by Elf November, who finds that one of the cores (Five) still has enough energy to materialize a body. This energy is transferred to number One, which gives Dezember back his body and plot relevance. Elf November hands the inactive soul of Rchimedes 1 to Dezember, saying that he will be able to revive the Demon Lord if he finds a compatible body to place it in, and suggests the body of Lake, Rchimedes' first son who is alive after being sent to the future.
Dezember obliges Elf's plan, trying to resurrect the Demon Lord through the body of Lake with debatable success. He attempts to educate Lake on the ways of the Demon Lord (with great enthusiasm from Dezember and very little from anyone else around him). When Lake is allowed to start a rampage in the area, it gets the attention of the heroes, who dispatch him.
Distraught at the loss of Rchimedes' soul and the chance to create a new Demon Lord, Dezember is overcome with grief and transforms into a large shadow monster that is still intelligent but much more difficult to defeat than regular Dezember. In this state, he mutters that the reason he wanted to help Rchimedes' so badly is because Rchimedes created him by accident out of a mana borne of extreme sadness. Because Dezember was born from the Demon Lord's despair, he felt that his sole purpose in life must to be to make the demon lord happy at least once.
(Incidentally, Rchimedes' soul does state that he's happy to know his beloved wife and son are alive and happy, though he disapproves of the method. Dezember does not know this.)
Dezember is defeated and reverts to his humanoid form, and which point Guilty Justice prevents the others from killing him and runs off with his unconscious body.
- Shadow magic - Dezember can manifest and control shadows to form any shape he desires. The shadows are solid black and can be created at will without relying on or being obedient to ambient light. He uses them mainly to form the mask that obscures half his face. When he uses his shadows offensively, they generally take the form of a mass/wave of blades that emanate from his own body.
- Has the ability to physically split into 6 different personalities. These can be combined into one body so long as all 6 are alive and healthy. The different bodies also are all created via mana around a dice core, and thus can return to the core either when inactive or killed. Unless totally lacking all mana whatsoever, the cores can leech magic from surroundings to slowly regain health after being 'killed'.
- Some variety of shapeshifting, at least between a human form and the giant finger puppet monster he turns into in Book 4.
- "Dezember Zwolf" means "December 12" in German.
- He calls his magic 'Dunkelheit', which means 'Darkness' in German.
- He is the "darling" of Guilty Justice.
- He has odd tendencies toward politeness or caring, such as wanting to sit but refusing to because Justice was hanging off him and he didn't want to inconvenience her, and Juli and Neun are shown making themselves perfectly at home in his house like he's their weird new guardian.
- He looks oddly similar to Ross and Lake.