Jonathan, David's loyal friend, was the son of which king?

Answer: Saul

Jonathan was Saul's son, named first among Saul's sons in 1 Samuel 14:49. He later died with his father on Mount Gilboa.

“One day Jonathan son of Saul said to the young man bearing his armor, “Come, let us cross over to the Philistine outpost on the other side.” But Jonathan did not tell his father. Now the sons of Saul were Jonathan, Ishvi, and Malchishua. His two daughters were named Merab (his firstborn) and Michal (his younger daughter).”— 1 Samuel 14:1, 14:49 (BSB)