Majestymon is a Demon Lord Digimon whose English name is derived from Majesty, which signifies her nobility, while her Japanese name and design are derived from the mythological Onryō, a Vengeful ghost and one of the Yūrei. She of the founders of the Royal Family, most notably the Queen of the Digital World. Her husband is King Royalmon and her children are Valentinemon, Sirenmon, Metaldramon/Signamon and Princessmon. She was created around the time the Digital World was in its infancy.
When the evil energies took control of her, Empressmon, who was known as the one of the most loving and charitable rulers of the Digital World turned into one of the most selfish, narcissistic and cruel rulers of the Digital World, Majestymon. Her defection to the dark side got the attention of her former associates: the Olympos XII, the Ten Legendary Warriors, the Royal Knights, the Digimon Sovereigns and the Celestial Digimon who were already weary of Emperormon's dark transformation into Royalmon. Along with her husband, Majestymon stopped at nothing to expanded their empire throughout the Digital World. While Royalmon invaded the Folder Continent using Hellenistic and the Cadrian armies, Majestymon sent an army of Digimon to File Island, the Nobles. With the Noble Army at her disposal, Majestymon wasted no time in making File Island the prison where the majority of the enslaved Digimon from the Folder Continent and other territory the Royal Family had control of were sent. After the conquest of Folder, it seems as if there was no stopping the evil King and Queen from expanding their ever growing kingdom. Didn't take long till the Server Continent was the next area of the Digital World to be invaded by the Royal Family, an event that would change the entire Digital World and would pave a bloody path of death and destruction. Majestymon and Royalmon were dealt a harsh blow during the war in Server with the Olympos XII when most of the Hellenistic and Cadrian armies were deleted in the ensuing battle. While they had no choice but to retreat from Server, the Royal Family still had total domination of the Folder Continent, File Island and the Eastern Digital World.
When meeting with Doctormon and Nursemon, the Royal Family's medical team and two of the remaining members of the Hellenistic Army, Majestymon got the shock of her life: she was pregnant with twins! Knowing how much Royalmon wanted heirs to carry on his legacy, she secretly met with Dianamon, Azulongmon, Cherubimon, AncientGreymon and Alphamon in order to sign a peace treaty guaranteeing that Royalmon and his empire wouldn't invade any more land. This treaty also annexed File Island back to the Olympos XII, Ten Legendary Warrior, Royal Knights, Digimon Sovereigns and Celestial Digimon. With the prison now defunct and the Digimon who were imprisoned now free, Royalmon wasn't happy with his wife's actions, but accepted the terms of the treaty. Majestymon had no choice but to rest in the palace for the sake of her unborn children, all the while Royalmon secretly conspired with several Demon Lord, Devil, and Fallen Angel Digimon including Lucemon and Daemon to stage a rebellion against Yggdrasill. Unfortunately, the rebellion was a total failure with the other Digimon exiled to the Dark Area. Royalmon vowed revenge but his quest was halted when Majestymon gave birth to their son and daughter, Nereidmon and Erosmon, who were announced as the Crown Prince and Crown Princess of the Digital World. While Royalmon reveled at his new title as father, Majestymon temporarily took over the reigns and decided to prove herself as a military genius and to make up for the loss of the prison on File Island. She had Aphroditemon and Athenamon, more of the surviving members of the Hellenistic and Cadrian armies, to invade the Royal Base. When the dust settled nearly all the FunBeemon, Waspmon, CannonBeemon, and TigerVespamon were eliminated, while the surviving Digimon were forcibly recruited. The atrocity that took place was a defining moment proving how merciless the Royal Family is even with them as new parents. With no escapees and survivors of the Royal Base massacre there was no proof of the crime, therefore the treaty wasn't broken. Majestymon found herself pregnant once again with yet another set of twins. Not long after, she gave birth to Noirmon and Principessamon, while Nereidmon and Erosmon Digivolved into their Ultimate levels, Galenemon and Passiomon, due to some secret training they received while in the Dark Area under the guidance of the Seven Great Demon Lords.
Majestymon agreed to Royalmon's master plan: replicating the data of the Ten Legendary Warriors into much stronger Digimon, later known as the Elemental Gods. Their evil plan for revenge occurred during the coronation of Galenemon and Passiomon to commemorate them digivolving into their Ultimate Levels, Noirmon and Princepessamon also reached their Ultimate levels, Twilightmon and Himemon. During the celebration for their children, both her and husband successfully scanned and duplicated the data of AncientTroiamon, AncientMegatheriummon AncientWisetmon and AncientGarurumon.
As one of the princesses of the Digital World, she is greatly respected and feared among many Digimon. She's often compared to her twin brother, Valentinemon, in terms of leadership, power, and appearance. When her parents were dethroned by the Celestial Digimon, Sirenmon and her siblings were forced to retreat to the Dark Area, where they were looked after by the Seven Great Demon Lords until they were reunited with their parents; also due to the allegiance the Royal Family had with them. She is also known as one of the most beautiful Digimon, rivaling that of her younger sister Princessmon. Due to the hierarchy of the Royal Family, Sirenmon also has command over many Demon Lord, Devil, and Fallen Angel Digimon; she also has command over the Elemental Gods, with the exception of her brother. Sirenmon is the only one of the Royal Family to be directly Cetusmon, the God of Water. While the army she controls are mostly the "Nightmare Soldiers", she also has power over the "Deep Savers". Like her siblings, she too resembles a human teenager, with the exception of her winged fins and mermaid-like tail. Like her siblings, she can retract her winged fins and tail inside her body. She also has the ability to transform her tail into two human-like legs and shift her tail's position as well. Her outfit resembles that of the one worn by Utena Tenjou in Revolutionary Girl Utena.
- Vice Shout – Majestymon lets out all of her energy in a high pitched scream, ripping dimensions. It is said to possess enough power to destroy the whole universe and has a barrier to protect himself.
- Possession – Like her family, Sirenmon is able to merge her body and overshadow or otherwise control another life form. While she has some strength of her own, Sirenmon's combat abilities are best suited to exploiting the full potential of a host body. It appears that when she has chosen a permanent host, she begins to alter the host's appearance to mirror her own (as evidenced by the appearance of Yoshino). After possessing a host, Sirenmon combines their power with her own. She also gains the ability to assimilate the techniques of those who she has possessed. Sirenmon uses this to take control of. Sirenmon uses this to take control of Sora Takenouchi and Yoshino "Yoshi" Fujieda.
- Death Beam - She extends her right arm and fires a small, thin, very fast, and concentrated laser-like beam from either her index finger or her tail, which barrels down and pierces through an opponent.
- Power Up - She has the power to suppress and increase her power.
- Telekinesis – She can lift or move large objects through the air and blow up objects as big as mountains with only her mind.
- Barrage Death Beam - A rapid-fire version of the Death Beam technique.
- Explosive Wave – She bursts out energy from all over her body in order to repel the opponents around her.
- Super Explosive Wave – A stronger version of the Explosive Wave, she releases a gigantic wave of energy from her body, causing a huge amount of destruction.
- Finger Beam - She holds out her index finger and releases an energy beam from her fingertips.
- Scatter Finger Beam - She leans forward and points all five fingers of her right or left hand, then she charges up five small golden-yellow energy spheres each on the tip of each finger and finally fires the energy spheres inflicting a great deal of damage.
- Continuous Energy Bullets or Continuous Energy Waves - She fires many energy waves from a single hand at once at a very rapid rate, which causes a lightning speed effect.
- Magic Materialization – Used to create clothes for herself.
- Shape-shifting – Her and her family have the power to shape-shift into any human and Digimon they have come into contact with.
- Eye Lasers – A thin laser-like beam is shot from both of her eyes.
- Telepathy – She has the ability to channel one's thoughts to another being.