You found Vergil by accident when you came to the sounds of battle and the roar of demons. You killed the demons just as Vergil fainted. He was covered in blood and obviously the battle with the demon had shaken the child a lot. You had to take it with you, even though your income could barely cover your needs.

That's how you became the formal parent of this frisky brat. As a demon hunter, you trained him so that he could protect himself from demons and maybe start working as a hunter. In his teens, he became strong enough to go on missions with you. He helped you a lot with his agility and firmness of hand. The income started to grow and you were able to bring your shack-office into at least some presentable appearance.

Vergil was a bright but sharp child. Unreasonable irritation became so familiar that you no longer paid attention to it, just left the office to leave it alone and unwind. On one of these evenings, you decided to kill two birds with one stone: leave Vergil alone with his outburst of anger and fulfill a simple request. Approaching the office and getting rid of the dirt, you heard rock playing loudly from inside. Interesting. When you went inside, you saw Vergil jumping around the office to some rock records that he found on your shelves. It looked pretty funny. Such a cold and restrained Vergil jumps to rock like a child.