Dante and Vergil, the Sons of Sparda.
The sons of Sparda are the twin sons of the Legendary Dark Knight, Sparda. Before his disappearance, Sparda fell in love with a human woman named Eva, who gave birth to identical twin sons, Vergil and Dante. Half-demon and half-human, the twins were born with supernatural abilities as well as white hair and blue eyes, all traits derived from their father's strong demon blood. Before Sparda disappeared, he gave his sons two powerful swords, each one being of his own creation with the swords themselves becoming the symbols of their spirits. The twins are considered to be among the most powerful cambions in the world as they are the offspring of a Knight of Hell.
The sons of Sparda spent their early childhood living together with their mother, Eva, after their father's disappearance. Vergil is the elder twin by two minutes and was always told from a young age to take care of his brother, Dante. Due to being the older brother, Vergil had much responsibility even at a young age, although this did not hinder the relationship between him and his family. After the disappearance of their father, Vergil was tasked as Dante's protector and, as a result, views himself as such. Thus whenever their mother would leave Vergil would watch over Dante. While Dante did not appreciate being spoiled, he nonetheless loves his brother greatly and hoped to one day return the favor. He would cook and care for Dante as well as protect him while their father or mother were out performing their duties.
Some time during the twins' childhoods, Eva was killed by demons. Although the details of this event were never clearly shown, Dante appeared to have been present during the attack and witnessed his mother's death (while mistakenly believing that his twin had also been killed). In reality, Vergil was separated from his brother and mother during the attack and ended up alone with only Yamato to aid him. This forceful separation and the trauma of being alone in the world may explain his cold and withdrawn nature later in life. Their mother's death was a huge cornerstone in the twins' lives and affected them deeply, leading the brothers to choose very different life paths. Dante dedicated the rest of his life to demon-hunting and protecting innocent humans, while Vergil, perhaps believing that humans (like his mother) died because they did not have the power to protect themselves, spent the rest of his life pursuing Sparda's power and trying to gain as much of it as possible.
Although, after Vergil's return from Hell thanks to his brother, it required much effort for the twins to make amends and reconnect. Vergil even abandoned his quest for power and focuses on increasing his own natural demonic power in order protect his friends and family. While their relationship can be strained from time to time, they nonetheless deeply care for one another and are willing to risk their lives to aid not only their allies but each other. However, there are numerous times where their ideologies can clash and thus pit them against each other because of it. For example, Dante would scold Vergil for not being careful when it comes to innocents being harmed during his battles and Vergil would perform the same when Dante would show mercy towards people who he considers to be worse than demons and so on. Despite being identical twins they are polar opposites in tastes, fighting styles, personalities and goals and a majority of time fighting and arguing with each other. They seem to represent the two sides of Sparda himself as he was known to be both rebellious like Dante and level-headed like Vergil, and his preferred color, purple, is the combination of Vergil's (blue) and Dante's (red).
While separately they are tremendously powerful in their own right, they are a virtually unstoppable force when together, as they have actively stopped and killed many high ranking demons, monsters, world-threatening entities and even gods either together or individually. Demons especially both despise and fear them for their reputation and status and even compared them to be the "Bogeymen" that demons and monsters have nightmares about. Lucifer himself stated that he never would have imagined that the brothers would give him so much trouble and even Satan noted that he is constantly surprised by how powerful the twins are. Mephistopheles, a high ranking demon himself and a Prince of hell, described them as "denim-wrapped nightmares" and should never be underestimated by anyone. The Great Old Ones even considered the brothers as enough of a threat to them and targeted them almost immediately after their release.