World Building- Notable Historic Figures
Cerberus-
If I were to sum up Cerberus, I'd say something like a cross between Godzilla and Hachiko.
A 30,000-year-old super monster, Cerberus was the strongest non-demon monster in history. Cerberus spent almost all of his life standing by the entrance to the Cursed Continent, the only safe passage in a ring of horrifying hazards that surround the land. While considered a symbol of unwavering duty and dedication to protecting monsterkind, in truth, Cerberus' reasons were more personal. Like the human that eventually ended his legend, Helen Arany, his motivations and personality were romanticised.
Cerberus was raised lovingly by the gentle-hearted monster sage, Plouton. Plouton was something of a political rival of the otherwise undisputed ruler of the Demon Empire, Emperor Baal. Plouton's peaceful desires with humans were considered radical, but his strength and wisdom granted him a great deal of respect. When Plouton set out with his retinue to try to broker peace with humanity, he asked Cerberus to stay behind, Plouton fearing he was too weak for the journey. No one ever found out what happened to Plouton- he simply never returned. Cerberus stayed by the gates for 30,000 years, training and growing, waiting for his master to return and dispatching any would-be intruders. Cerberus was given his name by Baal, as if he were a demon, in recognition of his achievements. Before this, he was known mostly as 'Growly' due to his anti-social attitude. Although he had many qualities that would qualify him to become a demon, Cerberus never underwent demonisation because the thing he wanted most was to see Plouton again- something that in his heart of hearts he knew would never happen.
Cerberus eventually died in battle with the strongest non-hero human in history, Helen Arany. She needed to cross the boundary of Hell to rescue Khan, the husband of Ece, the woman she loved. Their battle technically ended in a draw, as both were dying in the aftermath, but Helen was saved by Ece. As they lay dying, Helen with half her body burnt to cinders and Cerberus as a still-living severed head, the pair conversed and shared their mutual respect (between some frankly sad attempts to land a finishing blow). While enough monsters to form a town were born from his body, Guard Dog Town, only one emerged from the slime of that head- the monster that would become Naberius.
Of those few that knew them both, several characters mention that Rex and Cerberus have similar personalities. Conversely many feel, including Rex, that Naberius has a comparable nature to Helen. In a way, Helen and Cerberus were mirrors of each other. Cerberus spent his entire life in dedication to someone else, Helen lived only for herself. Helen cared only for the future, Cerberus only for the past. Cerberus was defensive, Helen aggressive. At the same time, they were similar. Both were romanticised and misunderstood by others, their indomitable will and astounding achievements assigned loftier motivations when they ultimately followed their hearts for arguably petty reasons.
By the time Helen faced him, Cerberus was 30 stories high, breathing fire, his head in the sky. (Godzilla! Godzilla! Godziiiiiila!) Despite commonly being referred to as a 3 headed dog, he actually had 4 heads- it's just that the 4th head happened to be his tail, which took the form of an enormous snake. He had a primarily black coat with white flecks. As befits a guard dog, his greatest strength was his nigh invulnerability, able to shrug off the efforts of entire armies and tanked Helen's signature meteor strikes. The Hellhounds that were created from the slime he left behind have similar bodies, but significantly smaller and less powerful. Naberius was unique from the start, possessing bird-like additions to his anatomy instead of snakish ones and being a pure white colour. Naberius is also very fluffy compared to his short-haired relatives.
Baal-
The one and only Demon Emperor, Baal is the self-appointed ruler of all monsterkind. Their influence has varied considerably over the years, occasionally accepted as the undisputed master of monsters. Though called an emperor, Baal is a genderless shapeshifter.
The oldest living demon, Baal was one of Sorrowman's successful experiments. With a personality similar to that of Apollyon, Sorrowman grew particularly attached to Baal and saw them as a proxy friend. Baal is well aware of this, and that Sorrowman's goals involve irrevocably devastating the planet to free Apollyon from the core of the world. They don't care- Baal's conviction to live is simply a dedication to amusing themselves, making it nearly impossible to destroy their will to live and make them mortal once more. Unfortunately for everyone, Baal cares only for exciting events and happily arranges for conflict between humans and monsters simply so they can observe them.
Possessing the ability to split into various bodies, pieces roam the world regularly. Baal has had a fragment of himself act as a close advisor to many Demon Lords without their knowledge. Naberius is no exception. A piece of Baal poses as the cat-themed monster Nyanastasia, one of Naberius' closest friends. When the time is right, Baal arranges for their alter-ego's 'death' to push Naberius into despair. They performed a similar trick as the spider monster Websley to trick Beelzebub into betraying Folio. When Rex takes the form of a Demon Lord, Baal attempts to infiltrate his forces as a toad monster named Annephibian, but having met Nyanastasia, Rex can identify their true identity.
Beelzebub
Beelzebub was the last Demon Lord before the cataclysmic war, indeed being one of the major figures that caused it.
Beelzebub began life as a simple fly monster. She emerged from the slime left behind when a monster community was slaughtered, amongst many others nearly identical to her. However, unlike her brothers and sisters who were self-serving and desperate to survive, she was deeply sentimental. She did her best to protect her siblings, but due to their weakness, all died one by one to various circumstances. When she was left alone, her powerful desire to protect her kind caused her to undergo demonisation. She established a community of monsters she protected diligently, eventually called to Hell to be given a task as the latest Demon Lord, and the name Beelzebub. In recognition of her extreme strength which exceeded the average Demon Lord, she was given the title Queen.
Demon Queen Beelzebub clashed against the Hero Folio Foster. Despite the circumstances of their lives, they were more similar than they were different. Both gentle but too devoted to their own kind. They could be described as no less than soulmates, and in what would have been the climactic battle between any other Hero and Demon Lord, they instead tearfully embraced in despair at what they had been forced to do and chose another path to peace. Together they were quite successful in leading both the Global State of humanity and the Demon Empire to reconcile, but Sorrowman and Baal tricked Beelzebub into believing Folio would betray her. She assassinated her closest friend and at long last passed the Hero Medal into Sorrowman's hands.
Beelzebub was soon filled with regret seeing the devastation the Cataclysm War brought and abandoned her people. In the process, she stole back the Hero Medal and placed it in the care of Khan, an elf warrior and member of Folio's party. In the modern-day, Beelzebub's conviction to protect those she cares for still burns on and she remains a demon. She's known as Queen B. and acts as the leader/mother to a small community of monsters that she raises in peaceful isolation.
Beelzebub possesses a charismatic, soft personality that charmed many people. Folio was vilified in history as the Fiend Folio who betrayed humanity after being seduced by Beelzebub or disparaged as the Fool Folio who was tricked by her lies. She is therefore historically attributed a penchant for lies and depicted as a sultry, scantily clad figure, as those who never met her could not understand how such charm could come from a particular hideous monster. Such depictions embarrass her endlessly as she's naturally quite bashful. Like many Demon Lords, she does have a humanoid form which is almost universally described as a 'modest, turtleneck sweater sort of beauty' without elaboration on what the heck that means. Sometimes 'she feels like your childhood best friend's mother' is added, again without explaining why.
Folio Foster-
The last Hero before the Cataclysm War, Folio is known in the modern-day as the Fiend Folio, the Fool Folio, the Failure Folio and various other alternative alliterative abuse.
Most people who never met him described Folio as 'pretty average, for a Hero.' Born in a small town, struck by tragedy in the early stages of his Demon Lord's plans, setting out on a quest to save the world, making friends, facing trials etc. However, Folio was humble and sceptical of the title of Hero, feeling that anyone could change the world. His words that 'the desire to save one person is all it takes to be a hero' have been passed down by those who did meet him- Rex learnt them from Helen, who learnt them from Ece, who learnt them from Khan, who learnt them from Folio. The phrase is also often repeated by the Hero Medal. Folio had an odd charisma that often turned enemies into friends- most of his party consisted of people who had once wanted to kill him. He also had a reputation as a ladykiller, which contributed to the mistaken belief Beelzebub was some sort of expert seductress. Folio was a friendly, loyal and courageous man who feared loneliness and letting people down more than anything.
The desperate circumstances of his death lead him to be the only Hero whose soul was not sealed into the Hero Medal when he died, but he rejected reincarnation. His soul has resided in the Cave of Spirits for the thousand or so years since the start of the Cataclysm War. It was where he and Beelzebub had their final battle and where he discarded his sword as a gesture of peace- it was the link with his sword that allowed him to remain in the mortal world. His faith in Beelzebub is strong enough he has believed all this time that she would never have betrayed him herself, correctly identifying that there was a sinister force behind the human-monster conflict but unable to discern what. He warns Rex of this when they meet and asks him should he ever meet Beelzebub to tell her he forgave her long ago. Folio also puts forward the theory that Rex's task as a hero is to save the monsters from their Folio's sword (which was just a normal, if very well made item) becomes Rex's legendary weapon, though due to their height difference the rapier is used as a magic wand. The time it spent in the Cave of Spirits, soaking up power from the crystals that grow there, make it an extremely potent spell casting catalyst.
Folio's strength was noted by his friends to vary considerably depending on his motivation and condition. At his best, when fighting with full conviction, he was an above-average Hero, but when his heart wasn't in it he was significantly below average.
Folio's animal theme is, like Rex, the domestic dog. However, while Rex represents a stray, Folio is a more traditional example. Social, faithful, playful and brave, but also easily overexcited perhaps too trusting. He was also, in significant contrast to Rex, a horndog.
Valefar-
Valefar is one of the mightiest warriors in history and the demon with the record for killed Heroes, at 15. Every time he fought a Hero, it was already after they had defeated their respective Demon Lord. In that respect, he is something like the optional super boss of demons. If he ever used his astounding power in the service of the Demon Empire, Valefar would have changed the course of history many times.
Instead, Valefar dedicated himself to battling only the strongest humans in the world. The only respect he showed for his race is that he didn't fight Demon Lords as well. The Cataclysm War was like paradise to Valefar, soaking himself in blood and battle that forged many warriors of the highest calibre. However, when it fizzled out, he was left aimless, unable to find challenges anymore. Valefar was thus overjoyed when he heard the news of Cerberus' defeat and scoured the world to find the one responsible. Valefar lost his will to live and became mortal when he witnessed Helen Arany, the last challenge on the planet, struggling to defeat small fry and landed the killing blow on her.
Valefar rapidly lost his strength and mostly accidentally became the head of a local monster band near Rex's hometown, slowly forming a bond with the weaklings of the area. When Rex killed them all as part of his preparations to leave, Valefar attempted to exact revenge but intended to die in the process. Battling against Mel Chilikov and Rex had him realise there was endless potential for the warriors he wanted to face in the world- just before death, he once more became a demon. Recovering his strength over numerous return battles, Valefar swapped from optional super-boss to recurring mini-boss, getting closer to his original power with each confrontation.