Though not as dramatic as being related to a hero, a villain being the hero's or one of the primary protagonist's former friend is a recurring trait in fiction. This is because the idea of a person the hero trusted as a friend turning on them can often have a great psychological effect on the hero and make them reluctant to fight that person. Frequently the friend joins the side of evil because of being brainwashed/possessed so as to be used against the hero, which makes the hero even more reluctant to fight them. These characters can also be on and off villains, changing between evil and good. Examples include Bloo and Kevin Levin. In these examples, when the Friend of the Hero is not being an evil villain, he or she can get along well with the hero.
On the flip side, this villain's betrayal or any other evil acts made the hero despise him/her and want nothing more than to kill their old friend.
This can also count for redeemed villains, who become friends with the heroes they fought after redeeming themselves.