I don't think the person Paul calls Kepha/Kephas is Simon Peter.

I know that in John 1:42 Simon is called Kephas and we're told that means Petros.  But the Protestant understanding of Petras and Petros is that Jesus is the Eben/Petra and all of us believers are Petron carved out of him.

Paul's Epistles use the name Peter only in Galatians, but he also uses Kephas once there.

In Galatians 2 he refers to Peter in verses 7 and 8, then to James, Cephas and John in verse 9, then to Peter again in verses 11 and 14.  Reading through the whole chapter it seems odd to me that he would this one time use a different name.

Now comparing to the Gospels, you might feel compelled to say Galatians 2:9 is like the verses that refer to Peter, James and John.  But Paul names them in a different order, in those verses Peter was always first.  And Paul is not contradicting Peter's primacy, the prior two verses just defined Peter as being in charge of bringing The Gospel to the Circumcision.  Also the James in this verse isn't the Son of Zebedee since it's about Acts 15 and he was Martyred in chapter 12.

It seems more likely that after already talking about Peter this verse is then listing other Pillars of the Church in Jerusalem.

The rest of Paul's references to Kephas are in 1 Corinthians.  1:12 and 3:22 refer to Paul himself, Apollos and Kepha being people who have fans in Corinth who are having fandom rivalries with each other, and Paul reminds them we're all supposed to be focused on Christ.

Chapter 15 is the most defining identifying characteristic of this Kepha however.   In verses 4 and 5 he says.

And that he was buried, and that he rose again the third day according to the scriptures: And that he was seen of Cephas, then of the twelve:

He didn't say "then the rest of the twelve".  Even without comparing this to other sources on the Resurrection, it clearly seems to be saying Kepha is not one of the Twelve.

Paul's list of Eye Witnesses to the Resurrection is not complete, but that who he does list is chronological is part of his point, he defines himself as the least of the apostles in part because he was the last eye witness to the Resurrection.

In The Gospel accounts, Peter does not see the risen Jesus till about the same time most of the Twelve do.  Peter enters the Empty Tomb, but that's not seeing the risen Jesus.  The defining characteristic of Kepha in 1 Corinthians 15:5 is being the first Eye Witness to the Resurrection.

Matthew 28 and Luke 24 tell us the Women who found the Tomb Empty saw the risen Jesus before the 12.  And Mark 16:9 and John 20 further clarify that the individual first eye witness of The Resurrection was Mary Magdalene.

Now I know this observation might seem like music to the ears of the DaVinci Code style conspiracy theorists, who'll say Paul is an older source so this is evidence that the Church later gave to Peter a title that belonged to Mary Magdalene.  But as I already pointed out, Paul does give primacy to Peter and discourages those who place too much importance on Kepha or himself or any other Apostle.

Since Magdalene probably comes from Migdol which means Tower, perhaps it refers to a stone tower?

Update March 27th 2024: I've now argued in favor of now viewing Mary as The Beloved Disciple.  

Also I've noticed that in 1 Corinthians 1:12 and 15:4-5 the Greek Textus Receptus says Cepha with no S on the end, that makes it arguably Grammatically Feminine.

I'm going to do something I used to be less willing to do and argue there has been textual corruption.  And then originally Cephas was only used in 1 John and all Paul's references are to Cepha.  Thus Cephas is Peter but Cepha is Mary.