Both brothers, Baul and Modeus, were mentored and trained by the Legendary Dark Knight Sparda. Modeus gave up swordsmanship for his brother, Baul, and Sparda left his power and will in the hands of Modeus. Baul vowed to harness Sparda's power and become stronger than his master, the Legendary Dark Knight. Both brothers became demon lords under Sparda's tutelage. They trained together with Sparda until the time that he rebelled against Mundus and left for the Human World; he commanded them to wait, but he never returned.
Baul eventually learned of the existence of Sparda's son, Dante, and vowed to kill him and take the blood of Sparda. At the same time, Modeus met Dante and apparently befriended him. However, when Baul eventually fought Dante, he was defeated, and Modeus was forced to fight Dante for revenge; he too was killed. Dante made a memorial to the two brothers, marked by their blades stabbed into a stone, but Sid found the grave and used Modeus's skull as part of his ritual to release Abigail.
Baul wears a white trenchcoat with a high collar lined with gold, white shirt, pants and greaves. He has a more serious and unfriendnly demeanor than his brother. His white hair is spiked up, and he has green eyes. In his demonic form, his trenchcoat disappears and his body is covered in a white, chitenous hide reminiscent of Dante and Vergil's Devil Triggers from Devil May Cry 3.
Modeus is clad in a black trenchcoat with a high collar also lined with gold, black shirt, pants, and greaves. His face is softer than Baul's, almost androgynus, and his eyes are gold. He has long dark hair and pointed ears, like an elf. His demonic form is very similar to his brother's, though he keeps his coat's tail, which resembles insect wings, much like Dante and Vergil's Devil Triggers.
Due to the training they received from Sparda, both brother's are feared and hailed by demons for their skill and power, with one demon claiming that when Baul lift his swords "A thousand demons die in an instant" and Baul was able to go toe-to-toe with Dante for several minutes before he was forced to reveal his true form.
Though Modeus was never seen fighting, Baul states that he was left with 'Sparda's true will'. It is unknown what this means.
Both brother's presumably possess the standard powers of demons, including enhanced strength, speed and endurance. It appears that Baul did not have enhanced regeneration for even after he revealed his true form, his wound from Dante did not heal.
Both brothers wield swords similar in design to Rebellion, Dante's main sword, though different in color and sharing a skull on the bottom of the hilt. Baul wielded two swords while Modeus used one.
Baul is an alternate spelling of Baal, which was said to be the name of an ancient Canaanite storm god, whom Beelzebub was said to be a bastardization of, and Modeus could be the short for Asmodeus, the demon of lust.