The First Instance of Devotedness

There are three striking instances in the life of Abishai that indicate that he was a man of faith and a true lover of David. The first instance is recorded in 1 Sam. 26:6, "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? And Abishai said, I will go down with thee." Abishai knew, of course, that Saul was only there because he sought David's life. To go down with David could only be at risk of his own life, and could only serve to subject himself to the animosity of the then-ruling king. Yet he goes with David, proving that devotedness to God's anointed, but rejected, king was the most important thing in his life.
What was it about David that had won this man's heart? Had he possibly been present, like Saul's son Jonathan, at the conquest of the Philistine? We know that he was David's nephew, the son of David's older sister Zeruiah. Yet, it would appear that he was at least the same age as David, and possibly a little older. But surely it was something more than the natural relationship that attracted Abishai's heart, for he is once and again set apart from his own brother Joab as having special affection for David.