Nahari was the armor bearer of David's nephew Joab. To use a golfing term, he was a caddy of the war kind. An armor bearer carried the soldiers weapons. I read they often carried the shield in front of the soldier. They were on the battle field and apparently Nahari took notes on the actions of war techniques of Joab because he was one of David's Mighty Men.
As you come across Joab's name in the Bible, keep in mind, when you read what Joab is doing, most likely Nahari was there witnessing it and/or supplying the right weapon for the job. Joab was a loyal confidant of David but they had a contentious relationship where David often overruled Joab's advice when it went against David's often clouded judgment.
Nahari was from Beeroth, a Canaanite town. During the time of Joshua, the residents of Beeroth formed an alliance with the Gibeonites and other surrounding areas and deceived Israel into making a covenant with them, even though God had told them not to. When Joshua found out they had been deceived, instead of killing them, he made them "woodcutters and water carriers for the community and for the altar of the Lord."
Nahari, possibly a child of a woodcutter or water carrier, and an armor bearer to the commander of the King's royal army became a Mighty Man under the leadership of King David.
N is for Nahari
For Joab's and Nahari's stories start at 2 Samuel 2.