Saul may not have known who the prophet Samuel was but Samuel knew who Saul was because the Lord had informed him on the previous day that Saul would come there and then identified him personally just before Saul spoke.
In the information that Samuel was given, the Lord said that he had a purpose of mercy for his people and a feeling of compassion towards them, and this despite their rebellion. Was this purpose of God realised in Saul’s experience? I think the answer is yes and no. He did defeat the Philistines at times but in the end the Philistines killed him. It was David who defeated them.
This raises an important aspect of God’s promises, and that is that often they are conditional on the recipient obeying his commands. Saul did not obey God’s commands and in judgment he lost in battle against those whom he would have defeated had he remained true to God.
After inviting him to the chief place at the meal Samuel speaks to Saul in a prophetic manner, saying three things: that he would describe Saul’s heart to him, that the donkeys had been found, and that Saul was the one Israel wanted to be their next king. Saul’s answer is rather deceptive because it contains a mixture of truth and untruths. Benjamin probably was the smallest tribe in Israel but Saul’s family certainly was not the least of the families in Benjamin.
What did Samuel mean when he said that he would tell Saul everything that was in his heart? He does not seem to mean Saul’s concern for the donkeys because Samuel assures Saul that they have been found. It is unlikely that Saul could have conceived he was in line for such a prominent role, so Samuel was not referring to that, and in any case he went on to tell Saul that he was the popular choice. I suspect Samuel wanted Saul to know the kind of man he was before he would become king.
This is an important aspect of preparation for any person who intends to serve God, all the more so if the role is a public one which will give opportunities for the person’s worse tendencies to show themselves, as they did in Saul. When the Lord used Samuel to highlight these features in Saul’s heart it was a sign of God’s gracious desire to help Saul overcome them. Similarly when the Lord uses one of his servants to point out defects in our hearts, often without the person knowing he is doing so, it is a sign of his grace.