Amasa was David's nephew, the son of Abigail, who was David's sister (1 Chr. 2:17). He was also a cousin to Joab, the commander of David's army.
Absalom appointed Amasa as commander of his rebel army, when he rebelled against David. After David's army defeated the rebel forces, and Absalom was killed by Joab, David forgave Amasa and appointed him commander of his army.
When David sent Amasa to mobilize the troops to chase after Sheba, he met up with Joab and Abishai, who were leading the king's bodyguard on the same mission. Amasa and Joab went to greet each other, but Joab pulled out a dagger and killed Amasa (2 Sam. 20:8-10).
Amasa was the one who convinced all the leaders of Judah to take back David as their king after the revolt of Absalom (2 Sam. 19:14).
Next person in the Bible: Amaziah, King of Judah