Atsu was born 2289 years ago as the son of Lucifer and a succubus; he is the second youngest of all of his brothers and sisters. How many siblings Atsu has isn't explicitly stated, but he is known to have at least three, Malthor, Nejoktu, and an unnamed sister. Only Nejoktu and his sister are completely his siblings, the rest all have different mothers.
Little is known about Atsu's early life before leaving Hell. At some point his father exiled two of Atsu's older brothers, including Nejoktu.
Despite the fact he, for half a millenia, Atsu was destined to become the new ruler of Hell. However, his father decided that, due to his lack of ambition, he would not carry out the purpose of hell. Instead, Lucifer gave over the throne of Hell to Atsu's younger brother
He was destined to become its next ruler by right of strength, however his father betrayed him and put his weaker brother on the throne due to Atsu's lack of a cruel nature. Enraged, Atsu left hell and went to the human world. In the process, he created a rift that pulled Jet Seyre in and sent him to another dimension.
Atsu was unseen for about a year afterwards. When he reappeared he met Jet. Atsu, unlike most demons, was quite human in emotion, and apologized to Jet for all that had happened. Jet was more than forgiving and asked him to join him. He accepted and traveled with him.
Despite the fact that he is indeed a demon, he holds distaste for them and nearly hates them, hates them for betraying him. Nonetheless, his name is both feared and worshiped throughout the world; although many would have his head.
Combat wise, Atsu is a force to be feared. Instead of using his spiritual energy to kill, he uses it so summon magical spells, allowing him to cast nearly infinite spells. However, when worse comes to worse, he can use his spiritual energy to completely devastate a battlefield, or to tear through an enemies. In hand to hand combat, Atsu almost always devastates opponents by hitting vital points, dislocating joints, or flat out killing opponents with single blows that are used in conjunction with burst of spiritual energy. A fighting style which eventually rubs off on Jet.