Jonathan was the eldest son of King Saul and the closest friend of David, recorded in 1 Samuel; he died alongside his father in the battle at Mount Gilboa.
1 Samuel 18:1 in the King James Version reads:
And it came to pass, when he had made an end of speaking unto Saul, that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul.
The verse appears in 1 Samuel 18, immediately after David's defeat of Goliath. Read it in context at 1 Samuel 18:1, or the full chapter at 1 Samuel 18. Encyclopedia entry: en.wikipedia.org/wiki/Jonathan_(1_Samuel).