Then answered David and said to Ahimelech the Hittite, and to Abishai the son of Zeruiah, brother to Joab, saying, Who will go down with me to Saul to the camp? Abishai said, I will go down with you.
"Abishai"  (1 Samuel 26:6), along with Joab and Asahel were children of Zeruiah, who according to  1 Chronicles 2:16 was a sister of David.  David, being the youngest in the family of Jesse probably had a number of cousins his own age or older.  "Abishai saved David's life in one of the Philistine wars  (2 Samuel 21:17), was implicated in the murder of Abner  (2 Samuel 3:30) and remained faithful to David during the rebellion of Absalom."[6]