Xelloss is a mazoku and the priest of Greater Beast Zelas Metallium. Another name for him is Jūshinkan, translated as Beast Priest. 1200 years ago when Hild sought out Ranma and Nabiki she ended up giving birth to two twins Xelloss and Sherra. Ranma never claimed them publicly due to same reason he couldn’t publicly claim his children during the Silver Millennium as well as the fact that a male chosen as consort to the Elder Goddesses fathering children with the Daimakaicho would have caused many problems.. Hild also asked not to inform Nabiki as the two wanted to make their own way without family power offering them an easy path.
However problems arose when after only Twenty Years the two of them to reach First Class Unlimited License. Something that no demon in the system ever made that in less than three centuries, and even then, they were making every dirty deal they could. Wanting both a challenge as well as to prove that their rise in power wasn’t just a fluke they asked their father to send them to Avalon. On Avalon, they would be nobodies, newbie demons looking for a start. Xelloss was recruited by Zelas during the Kōma War despite being away from Earth and unable to access the power base like the other New Gods can Xelloss became the most powerful of all the priests and generals of Ruby Eye Shabranigdu's subordinates.
In the Kōma War, Xelloss singlehandedly killed thousands of golden dragons; he is feared and hated by them ever since. They refer to him as the Dragon Slayer, although he does not like this name, probably because it does not fit the "image" he wants others to view him as. After the war, before serving Hellmaster, Xelloss's duty for a thousand years was to find stray copies of the Claire Bible and destroy them so that humans could not find an easy way to defeat mazoku (with Zanaffar), or utilize the power of the Lord of Nightmares. (This is not what he told Lina and her friends as he said he was a priest of a temple who wanted to get back a missing copy of the book.)
Xelloss posesses all the abilities of a high-ranking mazoku, including the ability to teleport with items or even people, and also the ability to maintain an incredibly detailed projected form, mimicking even the insides of the human body. He can also change his human form at will, switching his gender from male to female if he wishes. He has narrow purple eyes that are catlike, and his gaze very cold and sinister. This may very well be intentional: He usually keeps his eyes closed or at least slitted, along with wearing a cheerful expression, but when he opens them, his face changes from harmless to menacing in an instant. The clothing he appears to wear and his staff are all part of his physical manifestation and can become as incorporeal as the rest of him. Before he sold them to Lina, he possessed the Demon's Blood Talismans, four orbs that draw upon the power of all the ma-ō. He recieved these during the Kōma War as a reward from Shabranigdu.
Xelloss' actions can be interpreted in numerous ways; what he says is often at odds with what he does, and what he seems to do often appear at odds with the final outcome. Being Greater Beast's servant, Xelloss is her messenger and the medium through which she acts. He never disobeys the direct orders of his superiors, even when he does not actually agree with what they want him to do. Examples of this include when he did not think Valgarv would be swayed to Shabranigdu's side but asked him anyway, and also when Hellmaster Fibrizo borrowed him from his creator in order to protect Lina Inverse so that she would be able to cast the Giga Slave. Evidence suggests, however, that Xelloss likes to and often will exploit whatever small give he has in his orders, in order to fulfill his own desires. This was especially apparent in his assignment with Fibrizo, when he was forced to follow a plan he seemed to dislike entirely.
Xelloss often introduces himself as the Trickster Priest for good reason - he will mislead or cheat in order to get his purpose accomplished. (He himself stated that he never outright lies on principle; his clues usually have at least a fraction of truth in them.) Often, he will play pranks just to see the reactions of those around him, or manipulate others to do the work for him seemingly just because he can. For example he was the one who spreed the legend of the Jade Dragon the being who split Ruby-Eye Shabranigdo into seven pieces. A nickname his nephew thoroughly hates. His famous catchphrase is "Sore wa himitsu desu" literally "That is a secret". Despite his enormous powers, he prefers subtlety and manipulation over violence, although when Xelloss does enter a battle, he is known to be a ruthless opponent.
Whatever happens, Xelloss always remains polite, referring to everyone, even his enemies, as (name)-san. He also appears to have a certain sense of humor, which shows even in situations where posing as a human is not strictly necessary.
Xellos is the son of Ranma Kuramitsu and Hild like with his sister Urb Xellos’ unique nature causes a lot of inner conflict as his Demon and Human personalities are always in constant conflict inside him. While he doesn’t hate the part of him that came from his father—especially that part which put him in line for so many scenes of chaos—but at times it really makes being a demon a difficult job.
This is most likely the reason for the mutual respect he has for his niece Lina, although it might be a bit of an overstatement to say that they are actually friends. Xelloss seems to enjoy the chaos that follows Lina wherever she goes, and was often seen following her around, even though he had not been instructed to do so. When under orders, Xelloss usually uses Lina's reputation, powers, and personality in order to advance his own agenda, though such often leaves him on the receiving side of a vicious headlock.
Xellos seems to have this same love hate relationship with all of his family he never bothered to resurrect his sister until his mother requested that they return to earth. However he did warn her that sealing her memories was a bad idea and was disgusted by how his sister was treated by Dynast Grausherra. He also went out of his way to spread the Name Jade Dragon on both Avalon and Edo. Though he does seem to have a healthy fear and respect for his mother.