After David had finished talking with Saul, Jonathan became one in spirit with David, and he loved him as himself.
References for 1 Samuel 18:1
From that day Saul kept David with him and did not let him return home to his family.
And Jonathan made a covenant with David because he loved him as himself.
References for 1 Samuel 18:3
Jonathan took off the robe he was wearing and gave it to David, along with his tunic, and even his sword, his bow and his belt.
References for 1 Samuel 18:4