Jether

Abigail was the mother of Amasa, whose father was Jether the Ishmaelite.

Moreover, you know what Joab son of Zeruiah did to me—what he did to Abner son of Ner and Amasa son of Jether, the two commanders of the armies of Israel. He killed them in peacetime to avenge the blood of war. He stained with the blood of war the belt around his waist and the sandals on his feet.

The LORD will bring his bloodshed back upon his own head, for without the knowledge of my father David he struck down two men more righteous and better than he when he put to the sword Abner son of Ner, commander of Israel’s army, and Amasa son of Jether, commander of Judah’s army.

Absalom had appointed Amasa over the army in place of Joab. Amasa was the son of a man named Ithra, the Ishmaelite who had married Abigail, the daughter of Nahash and sister of Zeruiah the mother of Joab.