What was the relationship between Benjamin and Saul?
Saul, the first king of Israel, was from the tribe of Benjamin. 1 Samuel 9:1-2, 21
What the Bible says
1 Samuel 9:21
And Saul answering, said: Am not I a son of Jemini of the least tribe of Israel, and my kindred the last among all the families of the tribe of Benjamin? Why then hast thou spoken this word to me?
Douay-Rheims Bible (Challoner revision), public domain.