Joab

Definition:

Joab was an important military leader for King David throughout David's entire reign.

Before David became king, Joab had already been one of his loyal followers.
Later, during David's reign as king over Israel, Joab became the commander of King David's army.
Joab was also King David's nephew, since his mother was one of David's sisters.
When David's son Absalom betrayed him by trying to take over his kingship, Joab killed Absalom in order to protect the king.
Joab was a very aggressive fighter and killed many people who were enemies of Israel.

(See also: Absalom, David)