I believe it was Samuel and the woman is our biggest clue.
1: A subtle indication is the way "woman" is used in these particular verses.
In 28:11 the woman is הָֽאִשָּׁ֔ה......which refers to the witch of endor or a female servant of a demon.
A subtle change comes in 28:21, the woman is now הָֽאִשָּׁה֙......which is a respectful term for "woman"
2: In the middle of the meeting the woman "sees" clearly that it is Saul. The demon is no longer blinding her. The Demon is no longer controlling her, why? She became a believer because she saw the power of God and the Demon had to leave.
3: Look what the woman does after the meeting.
20Then Saul immediately fell full length upon the ground and was very afraid because of the words of Samuel; also there was no strength in him, for he had eaten no food all day and all night.
21The woman came to Saul and saw that he was terrified, and said to him, “Behold, your maidservant has obeyed you, and I have taken my life in my hand and have listened to your words which you spoke to me.
22“So now also, please listen to the voice of your maidservant, and let me set a piece of bread before you that
you may eat and have strength when you go on
your way.”
23But he refused and said, “I will not eat.” However, his servants together with the woman urged him, and he listened to them. So he arose from the ground and sat on the bed.
24The woman had a fattened calf in the house, and she quickly slaughtered it(Christ); and she took flour, kneaded it and baked unleavened bread from it. 25She brought it before Saul and his servants, and they ate. Then they arose and went away that night.
She went from the "witch" to Sauls maidservant. And Prepared Sauls "last supper."
I believe this was all Gods Grace, the woman Got saved and Saul had His last chance to see this Grace and make the choice to return.
Saul Died the sin unto death for reversionistic believers. He is not in the Lake of fire. He chose not to live in Gods plan and God took him home so someone else could do His job God had planned for Him.