The friendship between David and Jonathan was a covenantal relationship. There was a Godly connection between them.

"As soon as he had finished speaking to Saul, the soul of Jonathan was knit to the soul of David, and Jonathan loved him as his own soul." 1 Samuel 18:1

David and Jonathan forming an agreement. In this agreement, Jonathan was to be second in command in David’s future reign, and David was to protect Jonathan’s family